Hughes 
iieport  on  photo-electricity 


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Vol.  2.    Part  2  APRIL,  1921  Number  10 


BULLETIN 

OF  THE 

NATIONAL  RESEARCH 
COUNCIL 

REPORT  ON  PHOTO-ELECTRICITY 

Including  Ionizing  and  Radiating  Potentials  and 
Related  Effects 

BY  ARTHUR  LLEWELYN  HUGHES 

Research  Professor  of  Physics,  Queen's  University 
Kingston,  Canada 


PUBLISHED  BY  THE  NATIONAL  RESEARCH  COUNCIL 

OP 

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BULLETIN 

OF  THE 

NATIONAL  RESEARCH  COUNCIL 

Vol.  2,  Part  2  APRIL,  1921  Number  10 


REPORT  ON  PHOTO-ELECTRICITY 
Including  Ionizing  and  Radiating  Potentials  and  Related   Effects 

BY  ARTHUR  LLEWELYN  HUGHES 
Research  Professor  of  Physics,  Queen's  University,  Kingston,  Canada 

PREFACE 

The  writing  of  this  report  on  the  recent  progress  in  photo- 
electricity was  suggested  by  the  Photo-Electric  Committee  of 
the  Division  of  Physical  Sciences  of  the  National  Research  Council. 
The  author  wishes  to  express  his  appreciation  of  the  valuable 
suggestions  given  by  the  other  members  of  the  Committee,  viz., 
Professors  R.  A.  Millikan,  K.  T.  Compton,  J.  Kunz,  and  C.  E.- 
Mendenhall,  to  whom  the  report  was  sent  for  comment.  Thanks 
are  also  due  to  Professors  Compton  and  Mendenhall  for  supplying 
periodicals  and  summaries  of  articles,  not  accessible  to  the  author, 
and  to  the  authorities  of  Queen's  University  for  giving  ample 
opportunity  for  the  preparation  of  the  report. 

A.  LL.  HUGHES 

QUEEN'S  UNIVERSITY, 

KINGSTON,  CANADA, 

January,  1921 

CONTENTS 

I.     Introduction 84 

II.     lonization  of  gases  and  vapors  by  light 86 

III.  The  energy  of  photo-electrons 90 

Method  of  measurement  of  velocities 92 

Experimental  results 93 

The  photo-electric  threshold 96 

Fluctuations  in  the  photo-electric  threshold.     Dependence  on  the 

contact  difference  of  potential 99 

IV.  Total  photo-electric  effect 104 

Photo-electric  current  and  light  intensity 104 

Photo-electric  photometry 107 


84  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

V.     The  photo-electric  effect  as  a  function  of  the  frequency 

and  state  of  polarization  of  the  light no 

Richardson's  statistical  theory HO 

Experimental  results 112 

Normal  and  selective  effects 115 

VI.     Photo-electric  properties  of  thin  films 119 

VII.     Photo-electric  effects  of  non-metallic  elements  and  in- 
organic compounds 122 

VIII.     Photo-electric  effects  of  dyes,  fluorescent  and  phos- 
phorescent substances 124 

IX.     Positive  rays  produced  by  light 125 

X.     Sources  of  light  used  in  photo-electric  experiments. ...       125 
XI.     Ionizing  and  radiating  potentials 

Experimental  methods 1 27 

Collected  results 135 

Bohr's  theory  for  hydrogen  and  helium 139 

Spectral  series  notation ]  50 

Single  and  multiple  line  spectra 153 

Cumulative  effects — low  voltage  arcs 158 

Compound  gases 165 

ABBREVIATIONS  FOR  JOURNALS 

P.  R.  Physical  Review. 

N.  A.  S.  P.  National  Academy  of  Sciences,  Proceedings. 

A.  P.  J.  Astrophysical  Journal. 

J.  O.  S.  A.  Journal  of  the  Optical  Society  of  America. 

J.  A.  C.  S.  Journal  of  the  American  Chemical  Society. 

P-  M.  Philosophical  Magazine. 

Proceedings  of  the  Royal  Society  of  London. 

P.  L.  P.  S.  Proceedings  of  the  London  Physical  Society. 

A.  d.  P.  Annalen  der  Physik. 

P.  Z.  Physikalische  Zeitschrift. 

V.  d.  D.  P.  G.  Verhandlungen  der  Deutsche  Physikalische  Gesellschaft. 

Z.  f.  P.  Zeitschrift  fiir  Physik. 

C.  R.  Comptes  Rendus. 

CHAPTER  I 
INTRODUCTION 

The  principal  monographs  which  have  hitherto   appeared   on 
photo-electricity  are  the  following: 

R.  Ladenburg:  In  the  Jahrbuch  fiir  Radioaktivitat ,  1909. 
H.  S.  Allen:  "Photo-Electricity"  (Longmans),  1913. 

-  LI.  Hughes:  "Photo-Electricity"  (Cambridge  University  Press),  1914. 

.  Pohl  and  P.  Pringsheim:  "Die  Hchtelektrische  Erscheinungen"  (Vieweg)    1914 
*.  v^  Schweidler:  "Photo-Elecktmitat"  (in  Graetz'  "Handbuch  der  Elek'trizitat 
mid  des  Magnetismuss"  Earth),  1914. 

\V.  Hallwachs:  "Die  Uchtelektrizitat"  (published  privately). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  85 

The  present  report  gives  an  account  of  the  progress  which  has 
been  made  since  1913.  It  may,  therefore,  be  regarded  -as  supple- 
menting, and  bringing  up  to  date,  Allen's  "Photo-Electricity" 
and  Hughes 's  "Photo-Electricity."  The  division  into  chapters 
is  that  followed  in  the  author's  "Photo-Electricity."  A  chapter 
has  been  added  on  the  new  and  most  interesting  group  of  investi- 
gations which  may,  for  want  of  a  shorter  title,  be  designated  as 
"Ionizing  and  Radiating  Potentials  and  Related  Effects. "  This 
subject,  which  in  many  aspects  is  so  closely  related  to  photo- 
electricity that  it  is  natural  to  consider  it  a  part  of  photo-elec- 
tricity in  an  enlarged  sense,  has  attracted  an  immense  amount  of 
attention  from  both  theoretical  and  experimental  physicists  in 
recent  years.  Certain  investigations,  frequently  designated  as 
photo-electrical,  because  they  deal  with  the  change  of  resistance 
of  materials  like  selenium  under  illumination,  have  not  been  touched 
upon.  These  investigations  seem  to  have  little  in  common  with 
those  dealt  with  in  the  report;  it  would  be  well  if  some  such  title 
as  "photo-resistance  effects"  could  be  used  for  them  to  dis- 
tinguish them  from  the  effects  commonly  classed  as  photo-electric 
effects. 

Perrin  has  recently  published  a  remarkable  paper1  in  which  he 
suggests  the  important  and  possible  decisive  r61e  played  by  radia- 
tions in  determining  chemical  reactions,  fluorescence  and  phos- 
phorescence, radioactivity,  cosmical  evolution  and  changes  of 
state.  According  to  this  theory,  the  energetics  of  every  change 
in  the  configuration  of  a  structure  from  state  A  to  state  A'  may 
be  represented  by  equations  of  the  type  A  -f-  hv  =  A'  +  hv' , 
where  v  and  v'  are  the  radiation  frequencies  which  change  A  to 
A'  and  A'  into  A,  respectively,  and  h(v  —  v'}  is  the  heat  of  reac- 
tion. There  is  considerable  direct  support  for  this  theory  and 
it  leads  to  a  satisfactory  explanation  of  certain  facts  of  velocity 
of  chemical  reactions  which  were  not  explained  on  the  older  kinetic 
theory.  Recent  calculations  by  Langmuir2  prove  that  Perrin's 
theory  cannot  be  applied  to  many  cases  of  molecular  dissociation 
in  the  simple  form  proposed,  but  suggest  some  additional  source 
of  energy  for  which  the  light,  or  the  collision,  may  act  as  a  releasing 
"trigger."  Yet  there  is,  in  the  theory,  a  suggestion  that  photo- 
electric action,  in  an  enlarged  sense,  may  be  one  of  the  most  fun- 
damental and  important  occurrences  in  nature. 

lA.d.  P.,  n,5-108  (1919). 

2  J.  A.  C.  S.,  42,  2190  (1920). 


86  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

Throughout  the  report,  it  will  be  often  necessary  to  make  use 
of  the  quantum  relation  Ve  =  hv  =  he/*.  It  is  generally  con- 
venient to  express  V  in  volts  and  X  in  Angstrom  units,  in  which 
case  the  numerical  relation  between  them  is  given  by : 

_  ((1547  X  10-"  X  2.9986  X  10")  x  ^^  x  10« 


4.774  X  II)-10  X 

12331 
X 


CHAPTER  II 
IONIZATION  OF  GASES  AND  VAPORS  BY  LIGHT 

Comparatively  little  has  been  done  on  this  subject  since  1913, 
although  it  was  obvious  that  the  subject  was  very  incompletely 
covered.  The  technical  difficulties  are  very  formidable.  In 
the  first  place,  most  gases,  if  they  can  be  ionized  at  all,  require 
light  of  such  short  wave-length  to  effect  ionization,  that  very 
special  arrangements  have  to  be  made  to  secure  the  right  kind  of 
light.  Fluorite,  the  most  transparent  of  known  substances,  begins 
to  absorb  strongly  just  in  the  region  of  wave-lengths  where  ioniza- 
tion of  air  begins.  As  it  is  desirable  to  have  a  window  between 
the  source  of  light  and  the  ionization  chamber,  it  becomes  very 
difficult  to  find  a  window  transparent  to  the  active  light.  Another 
obstacle  to  securing  reliable  results  is  that  in  the  region  where  the 
ionization  of  gases  begins,  the  photo-electric  effect  of  metals  is 
enormous,  and  it  becomes  a  problem  how  to  disentangle  the  small 
ionization  in  the  gas  from  the  very  large  photo-electric  effect  of 
the  electrodes  (even  when  apparently  well  shielded)  which  are 
generally  necessary  to  separate  the  ions  in  a  gas. 

Ionization  of  Air.  Hughes1  had  obtained  results  which  indicated 
that  air  could  only  be  ionized  by  wave-lengths  shorter  than  about 
X  1350.  From  time  to  time,  various  investigators  (e.  g.,  Bloch2) 
had  obtained  results  which  seemed  to  show  that  a  weak  ionization 
of  air  could  be  obtained  by  intense  light  from  a  mercury  lamp 
(long  wave-length  limit  probably  X  1800).  In  view  of  the  spurious 
effects  of  slight  traces  of  impurities,  dust  particles,  nuclei,  etc., 
so  clearly  shown  in  the  extensive  investigations  of  Lenard  and 
Ramsauer,3  it  may  well  be  that  the  slight  ionization  observed  with 

1  Proc.  Camb.  Phil.  Soc.,  15,  483  (1910). 

«  C.  R.,  1912,  903,  1076. 

•  Ber.  d.  Heid.  Akad.,  1910-1911. 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  87 

ultraviolet  light  from  a  mercury  lamp  is  not  due  to  a  real  ionization 
of  the  air  molecules  themselves,  but  to  some  obscure  subsidiary 
effect. 

Ionization  of  Alkali  Vapors.  Theoretical  considerations  indicate 
that  the  vapors  of  the  alkali  metals  should  be  ionized  by  light  of 
much  longer  wave-length  than  is  necessary  to  ionize  air.  Against 
this  advantage  is  to  be  set  the  difficulties  of  working  with  the 
alkali  metals,  the  necessity  for  working  at  high  temperatures  to 
secure  an  appreciable  vapor  pressure,  and  the  fact  that,  if  the 
electrodes  become  covered  with  a  film  of  the  metal,  their  photo- 
electric effect  becomes  enormous  and  may  well  mask  any  ionization 
effect. 

Gilbreath1  obtained  results  which  were  interpreted  as  indicating 
a  true  ionization  of  potassium  vapor  at  temperatures  not  over 
65°  C.  The  light  used  was  that  from  an  arc,  or  from  a  500-watt 
lamp,  filtered  through  glass  (shortest  wave-length  transmitted, 
probably  about  X  3300).  Kunz  and  Williams2  recently  found  that 
caesium  vapor  was  ionized  by  light  of  wave-length  X  3190,  and 
that  wave-lengths  longer  than  this  were  quite  ineffective.  Special 
care  was  taken  to  ensure  absence  of  surface  effects. 

Discussion.  In  view  of  the  remarkable  utility  of  the  quantum 
theory  in  linking  up  facts  in  other  regions  of  photo-electricity  one 
naturally  looks  to  it  for  confirmation  of  some  of  the  results  obtained 
in  ionization  of  gases  by  light.  Other  things  being  equal,  one  may 
perhaps  be  justified  in  accepting,  tentatively  at  any  rate,  those 
results  which  link  up  best  with  the  quantum  theory,  rather  than 
those  which  have  no  obvious  connection. 

We  know  that  gas  molecules,  struck  by  electrons,  give  out 
radiation  when  the  energy  of  the  electrons  exceeds  a  certain 
critical  value  and  become  ionized  when  the  energy  exceeds  another 
critical  value.  The  potentials  associated  with  these  critical  values 
are  called  radiating  potentials  and  ionizing  potentials,  respectively. 
As  will  be  shown  in  a  later  chapter,  the  radiating  potential  VR 
is  related  to  the  frequency  VR  of  the  radiation  emitted,  as  follows  : 


when  h,  e,  m  and  VR  are  Planck's  constant,  the  charge  on  and  mass 
of  an  electron,  and  the  velocity  of  the  electron  acquired  from  the 
fall  of  potential  VR.  The  ionizing  potential  Vj  is  related  to  a 

1  P.  R.,  10,  166  (1917). 
-  To  be  published  shortly. 


88  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

certain  convergence  frequency  vi  in  the  spectrum  of  the  gas  by  a 
similar  equation, 


l 


Now,  in  view  of  the  results  in  the  X-ray  region,  we  should  rea- 
sonably expect  a  certain  interchangeability  in  the  effects  of  electron 
collision  and  radiation.  We  may  perhaps  expect  a  gas  to  be  ionized 
whether  it  is  struck  by  an  electron  moving  with  the  critical  velocity 
(corresponding  to  Vi)  or  illuminated  by  radiation  of  the  corre- 
sponding wave-length,  for  the  same  quantum  of  energy  is  involved 
in  both  cases.1  Thus,  in  mercury  vapor,  we  know  that  direct 
ionization  is  produced  by  electrons  of  velocity  V:  =  10.4  volts, 
which  corresponds  to  X  1188,  and  we  should  also  expect  that  radia- 
tion of  this  wave-length  would  be  effective  also.  This  is  too  far 
in  the  ultraviolet  to  allow  of  easy  verification. 

No  direct  ionization  of  a  primary  character,  of  mercury  vapor,  has 
been  definitely  proved  for  electrons  of  velocity  4.9  volts,  the  radiat- 
ing potential  corresponding  to  the  strong  line  X  2536  (though 
effects  associated  with  "low  voltage  arcs,"  to  be  discussed  later, 
show  that  a  strong  ionization  of  a  secondary  character  can  be 
obtained  even  here).  In  line  with  this  is  the  fact  that  when  mer- 
cury vapor  is  illuminated  by  light  of  wave-length  X  2536,  as  in 
R.  W.  Wood's2  experiments,  no  ionization  is  observed,  although 
the  illuminated  gas  re-emits  the  radiation  strongly.  Similarly, 
with  the  alkali  metals,  we  find  that  no  ionization  is  produced  by 
electrons  with  velocities  above  the  radiating  potential  but  below 
the  ionizing  potential,  and  we  should,  therefore,  expect  that  no 
ionization  should  be  produced  by  light  whose  frequency  is  less 
than  that  of  the  convergence  frequency,  or  limit,  of  the  principal 
series.  This  result  seems  to  be  borne  out  by  Kunz  and  Williams' 
recent  experiment.  The  ionizing  potential  and  radiating  potential 
for  caesium  are  3.9  volts  and  1.5  volts,  respectively,  which  corre- 
spond to  the  limit  (X  3191)  and  the  first  line  doublet  (X  8946  and 
X  8523),  of  the  principal  series  of  doublets  in  the  spectrum  of  Cs. 
Kunz  and  Williams  showed  that  caesium  vapor  only  began  to  be 
ionized  by  light  whose  wave-length  is  not  far  from  X  3191.  Should 

1  It  should,  however,  be  pointed  out  that  ultraviolet  light  of  frequency,  say  X  2536, 
incident  upon  most  metals  produces  a  copious  flow  of  electrons.  Electrons  having 
the  same  quantum  of  energy—  4.9  volts—  falling  on  the  metal  instead  of  the  radiation, 
do  not,  as  far  as  we  know,  produce  any  analogous  effect,  so  that  some  caution  perhaps 
is  to  be  observed  in  carrying  ideas  from  the  X-ray  region  to  the  ultraviolet  light  region. 

1  P.  M.,  23,  689  (1912). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  89 

this  view  be  correct,  it  is  difficult  to  account  for  Steubing's1  result 
that  mercury  vapor  can  be  ionized  by  light  from  a  mercury  lamp, 
i.  e.,  by  light  whose  wave-length  is  longer  than  X  1800,  and,  there- 
fore, far  longer  than  the  limit  X  1188  for  mercury.  It  is  certainly 
worth  while  investigating  carefully  where  ionization  of  mercury 
vapor  by  ultraviolet  light  sets  in,  as  Steubing's  criterion  for  dis- 
tinguishing between  surface  effects  and  true  ionization  of  the  vapor 
is  far  from  conclusive. 

Indirect  evidence  that  there  is  no  ionization  by  light  of  wave- 
length of  the  frequency  corresponding  to  the  radiating  potential, 
is  obtained  from  the  fact  that  when  mercury  vapor,  or  any  mon- 
atomic  gas,  is  subjected  to  bombardment  by  electrons  of  energy 
just  sufficient  to  call  out  the  radiation,  no  ionization  can  be  de- 
tected, although  molecules  are  being  illuminated  by  radiation 
from  their  neighbors.  (However,  one  should  bear  in  mind  some 
recent  work  of  Compton2  on  helium,  w^hich  indicates  that  a  gas 
under  the  influence  of  its  own  radiation,  when  sufficiently  intense, 
is  more  readily  ionized  by  electron  impact  than  in  its  normal  state.) 

Hughes's  results  on  air  indicated  that  air  is  ionized  by  light  of 
wave-lengths  shorter  than  X  1350,  corresponding  to  about  9  volts. 
Now  9  volts  is  not  far  from  the  values  7.5  volts  for  nitrogen  and 
9.5  volts  for  oxygen,  the  values  formerly  associated  with  the  ion- 
izing potentials.  Later  results  have  shown  that  7.5  volts  for  nitro- 
gen, however,  is  a  radiating  potential  rather  than  the  ionizing 
potential,  which  is  about  18  volts.  A  similar  test  has  not  been 
made  for  oxygen,  but  the  analogy  of  some  recent  experiments  on 
metallic  vapors  and  on  hydrogen  and  helium  would  imply  that 
the  9.5  volts  for  oxygen  was  the  radiating  potential  and  not  the 
ionizing  potential.3  Ionization  of  air  by  light  of  wave-length 
X  1350  apparently  then  contradicts  the  result  tentatively  arrived 
at,  that  the  wave-length  which  ionizes  air  corresponds  to  the  ion- 
izing potential  and  not  the  radiating  potential.  It  is  just  possible 

1  P.  Z.,  10,  787  (1909). 

2  P.  M.,  40,  553  (1920). 

3  Mohler  and  Foote  (Jour.  Opt.  Soc.  Amer.,  4,  49  (1920))  in  some  recent  experiments 
found  that  the  radiating  and  ionizing  potentials  for  Nj  are  8.18  and  16.9  volts,  for  O« 
7.91  and  15.5  volts  and  for  H2  10.4  and  13.3  volts,  thus  apparently  following  the  rule 
observed  for  monatomic  gases  that  the  lower  critical  potential  is  always  a  radiating 
potential.     However,  Franck,  Knipping  and  Kruger,  and  Compton  find  that  the  lower 
critical  potential  for  H2  is  definitely  an  ionizing  potential.     In  view  of  this,  one  may 
perhaps  require  further  proof  that  the  lower  critical  potential  for  diatomic  gases  is 
always  a  radiating  potential  as  appears  to  be  the  case  for  monatomic  gases. 


90  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

that  when  a  molecule  is  already  in  an  abnormal  state  due  to  the 
absorption  of  radiant  energy  of  wave-length  correponding  to  the 
radiating  potential,  further  radiant  energy  of  the  same  wave-length 
may  cause  it  to  be  ionized.  (This  speculation,  and  it  must  be 
recognized  merely  as  such,  implies,  in  other  words,  that  intense 
radiation  may  produce  ionization  when  feeble  radiation  would 
not.)  The  recent  work  of  Compton,  just  referred  to,  is  suggestive 
in  this  connection. 

It  is  recognized,  of  course,  that  the  effect  of  radiation  impinging 
on  an  atom  is  not  exactly  analogous  to  the  effect  of  a  collision  with 
an  electron,  for  the  absorption  of  radiation  is  probably  a  con- 
tinuous process,  while  the  absorption  of  energy  through  an  elec- 
tron collision  is  essentially  discontinuous.  Again,  from  the  experi- 
ments of  McLennan  and  others,  there  is  only  strong  absorption 
at  the  isolated  wave-lengths  of  the  principal  series,  whereas  elec- 
trons moving  with  any  velocity  between  the  radiating  potential 
and  the  ionizing  potential  give  rise  to  radiation. 

The  whole  subject  is  full  of  obscurities  and  suffers  from  lack 
of  experimental  data.  A  systematic  investigation,  with  improved 
methods,  of  the  ionization  of  gases  and  vapors  by  light,  should 
repay  investigation.  It  is  of  considerable  interest  to  ascertain 
definitely  whether  the  wave-length  of  the  light  which  ionizes  a 
gas  is  related  to  the  ionizing  potential  by  the  quantum  relation. 


CHAPTER  III 

THE  ENERGY  OF  PHOTO-ELECTRONS 

Exact  measurements  of  the  velocities  of  photo-electrons  are  'of 
the  utmost  importance  in  furnishing  evidence  for  testing  the  theories 
of  the  photo-electric  effect.  The  photo-electric  effect  was  among 
the  first  effects  to  which  the  quantum  theory  was  applied.  In 
1905,  Einstein  proposed  a  law  governing  the  relation  between  the 
maximum  emission  energy  of  the  photo-electron  and  the  frequency 
of  the  light  causing  its  emission.  Each  advance  in  experimental 
accuracy  has  given  a  more  accurate  verification  of  its  correctness. 
According  to  Einstein,  the  energy  of  a  photo-electron  ejected  from  a 
substance  by  light  of  frequency  v  is  equal  to  the  energy  associated 
with  a  quantum  of  light  of  that  frequency,  viz.,  hv,  less  the  loss 
of  energy  in  getting  out  of  the  substance.  The  relation  may  be 
written 

Ve  =  hv  —  V0e 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


01 


where  Ve  is  the  energy  of  the  electron  after  leaving  the  surface 
(V  being  the  voltage  necessary  to  stop  it),  and  V0e  is  the  energy 
lost  in  getting  out  of  the  substance. 

Some  implications  of  this  equation  are  as  follows: 

(1)  The  energy  of  a  photo-electron  is  a  linear  functon  of  the 
frequency  of  the  light. 

(2)  The  slope  of  the  line  connecting  the  energy  and  the  frequency 
is  the  well-known  quantum  constant  "h." 

(3)  The  long  wave-length  limit  of  the  photo-electric  effect,  or 
the  "photo-electric  threshold"    (to  introduce  a  convenient  term), 
is  that  for  which  the  electron  escapes  with  zero  energy.     Hence, 
the  frequency  v0  of  the  photo-electric  threshold  is  given  by 


so  that,  once  we  know  the  threshold  for  any  substance,  we  know 
the  work  necessary  to  pull  a  photo-electron  out  of  the  substance. 

Up  to  the  date  at  which  this  report  begins,  these  implications 
had  been  confirmed  through  the  work  of  Richardson  and  Compton 


Case 


Lighf 


Earth 


FIG.  1. 


and  of  Hughes.  In  view  of  the  importance  of  testing  the  implica- 
tions to  as  high  a  degree  of  accuracy  as  possible,  further  work  was 
necessary  as  the  researches  referred  to  yielded  a  value  of  "h"  from 
10  per  cent  to  20  per  cent  smaller  than  the  accepted  value,  and  the 
limited  range  of  wave-lengths  naturally  restricted  the  precision 
with  which  the  other  relations  could  be  tested.  (When  these 
results  were  obtained,  there  was  a  certain  amount  of  speculation 
as  to  whether  one  could  expect  complete  agreement  between  the 


92 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


photo-electric  value  and  the  theoretical  value  of  "h,"  as  it  was 
conceivable  that  the  slope  of  the  line  connecting  energy  of  photo- 
electrons  and  frequency  might  in  some  way  be  slightly  affected 
by  the  nature  of  the  substance  or  by  other  conditions.) 

METHOD  OF  MEASUREMENT  OF  VELOCITIES 

The  most  common  way  of  measuring  the  velocities  of  photo- 
electrons  is  to  measure  the  photo-electric  current  with  various 
Currenr 


r\cTarcf/f}<?  Par.      Q 


Accelerar/ny  POT. 


C  urrenT 


ReTard.  Pot 


FIG.  2. 


Acce/.  Pot. 


retarding  fields.  Consider  a  small  illuminated  surface  surrounded 
by  a  considerably  larger  conductor,  which  we  may  call  the  "case," 
as  in  fig.  1 .  The  photo-electric  currents  from  the  illuminated 
electrode  to  the  "case,"  as  functions  of  the  applied  accelerating 
or  retarding  potential,  take  the  form  shown  in  fig.  2a  (where  the 
saturation  currents  are  adjusted  to  the  same  value  for  each  wave- 
length). In  general,  the  curves  imply  that  while  some  electrons 
require  a  retarding  field  to  stop  them,  the  part  of  the  curves  between 
the  ordinates  at  O  and  A  apparently  indicates  that  some~photo- 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  93 

electrons  actually  need  a  small  accelerating  potential  to  pull  them 
out.  Richardson  and  Compton  were  the  first  to  interpret  this 
correctly.  The  actual  potential  difference  between  the  illuminated 
electrode  and  the  case  in  fig.  1  is  not  the  applied  potential  alone,  but 
this,  plus  or  minus  the  contact  difference  of  potential  between  the  case 
and  the  electrode.  When  this  is  correctly  allowed  for,  the  curves 
are  all  shifted  with  the  result  that  the  point  P  falls  accurately  on 
the  zero  ordinate,  fig.  26.  It  is  now  seen  that  the  photo-electrons 
have  emission  velocities  between  zero  and  a  maximum,  charac- 
teristic of  the  light,  and  that  saturation  is  obtained  as  soon  as 
the  retarding  potential  is  diminished  £/; 
to  zero,  provided  effects  due  to 
electrons  reflected  from  the  case  are 
guarded  against.  (The  displacement 
of  the  saturation  point  from  the 
zero  ordinate  may  often  be  a  con- 
venient, though  an  inaccurate,  way 
of  measuring  contact  potential  differ- 
ence as  Richardson  and  Compton 
pointed  out.)  On  plotting  the  max-  v0  Frequency 

imum  emission  energies  (corrected  for 

contact  potential  differences)  against  the  corresponding  frequencies, 
a  straight  line  as  shown  in  fig.  3  is  obtained.  The  slope  of,  this 
line  should,  according  to  Einstein's  equation,  give  "h,"  and  its 
intercept  with  the  abscissa,  the  photo-electric  threshold. 

EXPERIMENTAL  RESULTS 

Kadesch1  made  an  advance  in  that  he  used  Na  or  K  as  the 
illuminated  material  and  oxidized  copper  as  the  material  for  what 
we  have  termed  the  case,  fig.  1.  These  two  metals,  being  photo- 
electrically  active  in  the  visible  spectrum,  enable  one  to  use  a 
wide  range  of  wave-lengths  for  investigation.  It  is  difficult  to 
ensure  that  no  light  falls  on  the  inside  of  the  "case"  (either  by 
reflection  or  otherwise).  The  photo-electric  effect  produced  by 
this  light  causes  a  small  current  to  flow  in  the  opposite  direction 
to  the  one  under  investigation,  viz.,  that  from  the  illuminated 
electrode,  when  the  potential  is  such  as  to  retard  the  main  stream 
of  photo-electrons.  This  "return"  current  reduces  the  direct 
current  everywhere,  and  its  effect  becomes  the  more  marked  the 

1  P.  R.,  3,  367  (1914). 


94  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

smaller  the  direct  current.  Hence  it  is  that  just  about  the  point 
at  which  the  direct  current  vanishes  altogether,  i.  e.,  the  point 
giving  the  maximum  emission  energy,  an  incorrect  value  is  obtained. 
This  error  can  be  avoided  by  limiting  the  investigation  to  a  range 
of  wave-lengths  over  which  the  illuminated  substance  is  photo- 
electrically  sensitive,  and  over  which  the  "case"  is  insensitive. 
This  was  realized  in  Kadesch's  experiments,  as  the  photo-electric 
threshold  of  copper  oxide  is  far  in  the  ultraviolet,  while  the  sodium 
and  potassium  are  sensitive  in  the  visible  spectrum.  Kadesch 
found  the  linear  relation  to  hold  and  that  the  experiments  on  K 
and  Na  gave,  respectively,  the  values  of  6.16  X  10  ~27  and  6.09  X 
10-27  for  "/*." 

Millikan1  examined  the  question  with  great  thoroughness  and 
took  every  precaution  to  avoid  spurious  effects.  He  used  as  his 
sensitive  metals,  Li,  K  and  Na.  These  metals  were  so  mounted 
that  fresh  surfaces  could  be  cut  and  exposed  in  vacuo.  The  con- 
tact difference  of  potential  with  respect  to  the  "case"  was  measured 
in  the  same  vacuum  with  the  photo-electric  measurements  and 
almost  simultaneously  with  them,  so  that  there  could  be  no  error 
from  time  changes.  The  case  was  made  of  oxidized  copper  whose 
threshold  was  slightly  above  X  2536  in  one  set  of  experiments  and 
slightly  below  it  in  another  set.  Hence,  a  range  of  wave-lengths 
from  X  2536  to  about  X  5461  was  available  in  the  case  of  Na  and 
to  about  X  4339  in  the  case  of  Li,  free  from  the  possibility  of  a 
spurious  effect,  of  the  kind  referred  to  in  discussing  Kadesch's 
experiments. 

Another  source  of  spurious  effects  is  that  the  monochromator 
when  set  to  isolate  any  particular  wave-length  in  the  spectrum 
(of  a  mercury  arc,  for  example)  does  not  isolate  it  completely. 
On  account  of  internal  reflections  in  the  monochromator  of  one 
kind  or  another,  small  amounts  of  scattered  light  of  all  wave  lengths 
pass  out  along  with  that  portion  of  the  spectrum  which  the  mono- 
chromator is  set  to  isolate.  The  stray  light  of  shorter  wave-length 
than  that  with  which  the  investigator  is  working  is  troublesome 
in  that  it  causes  the  emission  of  electrons  of  energies  greater  than 
those  corresponding  to  the  wave-length  for  which  the  monochro- 
mator is  set.  The  stray  scattered  light  of  shorter  wave-length 
than  that  in  use  was  frequently  stopped  out  in  Millikan's  experi- 
ments by  means  of  light  filters  whose  transparency  ended  just 

J  P.  R.,  7,  355  (1916). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  95 

beyond  the  short  wave-length  side  of  that  part  of  the  spectrum 
which  the  monochromator  was  set  to  isolate.  Large  photo-electric 
currents  were  obtained  by  using  an  intense  source  of  light  and  by 
using  surfaces  freshly  prepared  in  vacuo.  This  enabled  the  points 
corresponding  to  the  maximum  emission  energies  to  be  determined 
with  much  precision.  It  was  concluded  that  the  value  of  "h" 
was  6.569  X  10~27  from  experiments  on  sodium  and  6.584  X10~27 
for  experiments  on  lithium.  The  final  value  for  "h"  is  given 
as  6.56  X  10~27  to  1/z  per  cent.  Millikan  also  verified  with  great 
accuracy  the  result  that  the  photo-electric  thresholds  of  the  metals 
were  given  by  the  intercept  of  the  line,  representing  the  linear 
relation  between  the  emission  energies  and  the  frequency,  and  the 
potential  axes  (Fig.  3),  due  allowance  being  made  for  contact 
potential  difference. 

Sabine1  attached  a  photo-electric  cell,  in  which  Zn,  Cd  and  Cu 
were  the  active  metals,  to  a  vacuum  spectroscope  which  enabled 
him  to  use  exceptionally  short  wave-lengths,  the  shortest  being 
X  1250.  By  combining  his  results  for  X  1250  with  those  of  other 
experimenters  for  longer  wave-lengths,  he  arrived  at  values  of 
"h"  fairly  close  to  the  accepted  values  (6.58,  6.71,  7.23  X  10~27). 
It  was  admitted  that  the  procedure  of  combining  results  in  this 
way  was  less  satisfactory  than  if  observations  on  all  wave-lengths 
had  been  taken  under  identical  conditions. 

Ramsauer2  investigated  the  emission  energies  of  photo-electrons 
by  means  of  a  method  different  from  the  retarding  electric  field 
method  usually  adopted.  The  photo-electrons  were  deflected 
by  a  magnetic  field  and  those  within  a  narrow  range  of  velocities 
were  sorted  out  by  suitably  placed  slits.  Thus  the  distribution 
of  velocities  could  be  determined.  A  great  drawback  of  this  method 
is  that  the  number  of  photo-electrons  available  after  being  sorted 
out  is  very  small  in  comparison  with  the  number  handled  in 
the  usual  electric  field  method.  Ramsauer  concluded  that  there 
is  no  such  thing  as  a  definite  maximum  emission  velocity  for 
photo-electrons,  and  also  that  the  distribution  of  energies  of 
the  photo-electrons  is  the  same  before  and  after  passing  through 
the  surface.  He  obtained  a  linear  relation  between  the  most 
probable  velocity  and  the  frequency  of  the  light.  With  regard 
to  his  contention  that  no  definite  maximum  emission  velocity 

1  P.  R.,  9,  210  (1917). 

*  A.  d.  P.,  45,  1120  (1914). 


F 


96  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

exists,  Millikan1  points  out  that  the  experimental  arrangements 
were  unsatisfactory,  chiefly  because  the  part  of  the  spectrum 
supposed  to  be  isolated  by  the  monochromator  was  contaminated 
by  considerable  amounts  of  light  of  shorter  wave-length.  A 
detailed  criticism  will  be  found  in  Millikan's  paper. 

THE  PHOTO-ELECTRIC  THRESHOLD 

Important  consequences  follow  from  a  consideration  and  ampli- 
fication of  Einstein's  equation.  Millikan  has  examined  it  in 
detail,  and  the  following  account  is  based  on  his  papers.2 

Consider  a  metallic  surface,  A,  emitting  electrons  when  illumi- 

nated  by  light  of  frequency  v.     The 

energy  of  the  fastest  of  these  elec- 
trons is  obtained  by  measuring  the 
retarding  potential,  V  volts,  necessary 
~  to  prevent  them  reaching  C.      The 
field  acting  on  the  electrons  includes 
the    contact   difference  of  potential 
between   C   and    A,    say    K    volts. 
~^~       ~  Hence 

-••* ^  (V  +  K)e  =  hv  —  p          (1) 

p*"  where  p  is  the  energy  lost  in  pass- 

ing from  the  system  where  the  photo- 
electron  starts,  to  a  point  just  outside  the  surface.  (A  is 
electro-positive  to  C,  if  K  assists  V.)  Experiments  (see  previous 
section)  have  shown  that  the  smallest  frequency  v0,  for  which  the 
surface  A  has  a  photo-electric  effect,  is  that  one  for  which  the 
electron  emerges  with  negligible  velocity  (i.  e.,  V  -f  K  =  O), 
and  it  then  follows,  since  O  =  hv0  —  p,  that 

(V  +  K>  =  hv  —  hv0.  (2) 

If  now  a  surface,  B,  be  substituted  for  A  and  illuminated  by  the 
same  light,  we  have  a  similar  equation 

(V  +  K>  =  hv  —  hv'0  (3) 

where  V,  K',  and  v0'  refer  to  the  stopping  potential,  the  contact 
potential  with  respect  to  the  case,  and  the  photo-electric  threshold 
of  the  surface  B.  Subtracting  (2)  from  (3)  we  get 

(K'  -  K)  =  h-  (,0  -  „;>  -  (V  -  V)  (4) 


1. 


P.  R.,  7,  18  (1916). 
/Wa.,  7,  18,355  (1916). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  97 

/contact  potential  \  _  ^(frequency  difference)       (  difference  between) 
\betweenAandBj       e\  of  thresholds  /     \  stopping  potentials /' 

This  equation  is  one  which  can  be  checked  up  by  experiment. 
Before  doing  so,  it  is  well  to  look  at  it  from  another  angle.  Let 
us  regard  p  in  equation  (1)  as  being  made  up  of  two  parts,  pi, 
the  energy  lost  when  the  electron  leaves  its  parent  system  and 
becomes  a  "free"  electron,  and  pz,  the  energy  lost  by  the  free  elec- 
tron in  passing  through  the  surface,  pt  may  be  conveniently 
written  as  Vse,  where  Vs  is  the  intrinsic  potential  of  the  metal. 
(Vse  is  probably  identical  with  the  energy  lost  by  an  electron  in 
passing  through  the  surface  in  the  thermionic  effect.)  Equations 
(2)  and  (3)  now  become 

(V  +  K>  =  hv  —  Vse  —  pl 
(V  +  K>  =  hv  —  V'se  —  p[ 
which  on  subtracting  yield 

(V  —  V)  +  (K'  —  K)  =  (Vs  —  Vs)  —  Pl  ~  pl. 

As  the  difference  between  the  intrinsic  potentials  (Vs  —  Vs') 
is  nothing  other  than  the  contact  difference  of  potential  (K'  —  K), 
we  get 

(V  -  v)  =  *L=A  (5) 

If  this  view  of  the  mechanism  of  the  photo-electric  effect  be  correct, 
then  a  careful  comparison  of  equation  (4)  with  the  experimental 
data  will  tell  us  whether  (V  -—  V)  for  two  metals  is  finite  or  zero, 
and  then  by  (5)  whether  any  difference  exists  in  the  pi's,  i.  e.,  in 
the  work  necessary  to  extract  the  photo-electron  from  the  parent 
system  and  make  it  a  "free"  electron. 

The  experimental  evidence  obtained  by  all  observers  is  now  in 
agreement  that  when  .any  two  different  metals  are  brought  into 
rapid  succession  before  a  Faraday  cylinder  into  which  they  dis- 
charge photo-electrons,  the  stopping  potential  for  a  given  value 
of  the  wave-length  incident  upon  them  is  exactly  the  same.  Hence, 
the  difference  (V  —  V)  in  each  case  is  shown  by  experiment  to 
be  zero,  and  equation  (4)  degenerates  into  the  following  equation:* 

*  The  stopping  potential  for  a  given  wave-length  is,  however,  often  observed  to  change 
with  time  as  in  Millikan's  experiments,  so  that  if  much  time  elapses  between  measure- 
ments of  the  stopping  potentials  for  two  different  metals,  the  experimental  equation 
will  have  the  form  of  (4)  and  the  observed  values  of  (V— V)  will  measure  a  contact 
1,  M.  F.  which  is  due  to  a  transient  surface  charge  rather  than  to  an  intrinsic  property 


98  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

K'-K  =  -(„„  —  r.0-  (6) 

e 

As  Millikan  points  out1  the  interpretation  of  equation  (6)  can 
only  be  that  the  energy  after  escape  of  the  electron  from  the  atom 
is  always  equal  to  hv,  the  absorbed  energy  being,  contrary  to  the 
physical  theory  which  guided  Einstein,  greater  than  hv,  or  else 
that  the  same  energy  is  required  to  detach  an  electron  from  all 
atoms  (an  impossible  conclusion)  or  else  that  the  photo-electrons 
are  from  the  beginning  the  free  electrons  rather  than  the  elec- 
tronic constituents  of  the  atoms.  The  first  of  these  conclusions  is 
exactly  the  one  to  which  Barkla  was  led  by  his  work  on  X-rays. 
On  the  other  hand,  the  last  conclusion  is  very  difficult  to  reconcile 
with  the  photo-electric  properties  of  insulators,  which  have  no 
free  electrons,  and  also  with  the  independence  of  photo-electric 
currents  of  the  temperature. 

Whichever  of  the  foregoing  interpretations  be  made  as  to  equa- 
tion (6),  it  requires  that  the  whole  of  the  energy  hv0  of  the  electron 
be  expended  in  passing  through  the  surface  and  hence  that  hv0 
is  a  measure  of  Vs,  the  intrinsic  potential  of  the  metal.  Now, 
Vs  is  given  by  thermionic  experiments,  and  V0  may  be  obtained 
from  the  photo-electric  thresholds  (V0<?  =  hv0}.  The  following 
is  a  table  drawn  up  for  comparison.  Unfortunately,  the  varia- 
tions in  the  values  for  the  thermionic  results  are  only  equalled 
by  the  uncertainty  in  the  photo-electric  quantities.  For  example, 
the  values  for  Vs  for  Pt  (taken  from  O.  W.  Richardson's  "The 
Emission  of  Electricity  from  Hot  Bodies")  range  from  4.1  to  6.6 

of  the  metal.  This,  as  Millikan  points  out,  enables  the  distinction  to  be  drawn  be- 
tween intrinsic,  or  true,  contact  E.  M.  F.'s  and  spurious  contact  E.  M.  F.'s  due  to  sur- 
face charges.  In  Millikan's  experiments  reported  in  1916  such  charges  developed  on 
the  copper  oxide  surface,  due  presumably  to  the  entanglement  of  free  electrons  in  this 
surface.  This  accounts  for  the  difference  which  was  found  between  the  measured 

contact  E.  M.  F.  between Na  and  CtiO,  i.  e.,  (K'  —  K),  and  -  (Vo  —  Vo')  as  measured 

for  the  same  substances.  In  later  experiments,  soon  to  be  published,  Na  and  U  were 
brought  in  rapid  succession  before  the  Faraday  cylinder  covered  with  CuO  and  it  was 
found  that  the  stopping  potentials  were  identical  for  the  two  metals.  The  static 
charge  on  the  CuO  was  the  same  in  both  measurements  and  therefore  was  eliminated. 
If,  however,  a  considerable  time  elapsed  between  the  times  of  testing  the  Na  and  the 
Li  so  that  the  static  charge  on  the  CuO  had  opportunity  to  change,  the  stopping  po 
tentials  were  found  to  be  different.  To  summarize  briefly,  Millikan  finds  that  equation 
(4)  is  always  correct,  whether  static  charges  are  present  in  the  CuO,  or  not,  but  when 
they  are  eliminated  equation  (6)  is  found  to  hold. 

1  P.  R.,  7,  26  (1916). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


99 


volts.  5  volts  is  taken  here  as  being  possibly  the  most  reliable 
value.  The  table  is  merely  given  to  illustrate  the  order  of  the 
correspondence.  The  evidence  is  quite  insufficient  to  warrant 
its  use  in  deciding  for,  or  against,  a  connection  between  the  two 
effects.  Further  investigation  is  required,  but  the  exrJ^miental 
difficulties  are  very  considerable.  At  best,  one  can  say  that  there 


Metal 

Pt 
W 

Photo-electric  threshold 

Thermionic 

vs 

X0 

V0 

2800 
2300 

(Richardson  and 
Compton) 
(Hagenow1) 

4.4  volts 
5.7  volts 

5.0  volts     (Richardson 
loc.  cit.) 
43  volts 

CuO 

2500 

(Millikan) 

5.0  volts 

1  9  volts 

is  only  the  same  order  of  values  for  Pt  and  W,  while  for  CuO  there 
is  a  considerable  difference.  Possibly  the  value  of  pi,  the  part  of 
the  energy  lost  in  detaching  the  photo-electron  from  its  parent 
system,  and  making  it  into  a  free  electron,  is  much  greater  for  a 
non-metal,  than  the  energy  lost  in  passing  through  the  surface, 
hence  the  big  discrepancy  for  CuO. 

FLUCTUATIONS  IN  THE  PHOTO-ELECTRIC  THRESHOLD.    DEPENDENCE 
ON  THE  CONTACT  DIFFERENCE  OF  POTENTIAL. 

At  the  beginning  of  the  period  covered  by  this  report,  it  was 
recognized  that  the  photo-electric  threshold  obtained  experi- 
mentally was  largely  determined  by  insignificant  surface  conditions, 
presumably  arising  from  re-acting  gas,  but  there  did  not  seem  to 
be  any  weighty  reasons  why  it  should  not  be  possible  to  arrive 
at  the  real  value  of  a  photo-electric  threshold  characteristic  of  the 
metal  under  investigation,  uninfluenced  by  fugitive  surface  con- 
ditions. The  photo-electric  thresholds,  and  through  them  the 
potentials  given  by  the  quantum  relation  for  a  large  number  of 
metals  might  well  yield  an  interesting  and  valuable  series  of  atomic 
constants.  However,  from  this  point  of  view,  the  experimental 
results  have  been  disappointing,  for  they  have  shown  clearly  how, 
time  after  time,  almost  imperceptible  changes  in  conditions  can 
affect  the  value  of  the  threshold  and  consequently  make  it  almost 
impossible  to  decide  which,  if  any,  of  the  many  experimental  values 

1  P.  R.,  13,  415  (1019). 


100  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

obtained  is  the  real  value  of  the  photo-electric  threshold  charac- 
teristic of  the  substance  under  examination. 

But  few  experiments  have  been  carried  out  on  the  shift  of  the 
photo-electric  threshold.  A  much  larger  number  of  investigations 
have  been  carried  out  on  the  effect  of  varying  the  conditions  on 
the  total  photo-electric  current.  There  is  no  doubt  but  that  con- 
ditions which  change  the  total  photo-electric  effect  of  a  substance 
(i.  e.,  its  photo-electric  sensitivity  to  unresolved  light)  are  almost 
always  due,  in  part,  to  a  shift  in  the  photo-electric  threshold. 
Thus  conditions  which  shift  the  photo-electric  threshold  towards 
the  red  end  of  the  spectrum  naturally  increase  the  total  photo- 
electric effect,  as  in  this  way  a  greater  portion  of  the  spectrum 
becomes  available  photo-electrically.  Unfortunately,  in  many 
cases,  not  sufficient  information  is  given  to  enable  one  to  infer 
what  happens  to  the  threshold  when  conditions  are  altered  so 
as  to  change  the  total  photo-electric  effect.  The  photo-electric 
threshold  being  a  more  definite  constant  than  the  total  photo- 
electric effect,  insofar  as  its  use  for  theoretical  discussions  goes, 
it  is  highly  desirable  that  observations  be  made  on  it  in  all  experi- 
ments on  variations  in  the  total  photo-electric  effect  whenever 
possible.  In  the  following  discussion,  only  those  investigations 
from  which  inferences  as  to  the  shift  of  the  photo-electric  threshold 
can  be  made,  are  considered. 

Pohl  and  Pringsheim1  found  that  the  photo-electric  threshold  for 
new  metallic  surfaces  (generally  obtained  by  distillation  of  the 
metal  in  vacuo)  moved  spontaneously  in  the  course  of  a  few  hours 
over  a  range  of  an  octave  of  frequencies  or  more.  Thus  the  photo- 
electric threshold  for  a  calcium  amalgam  moved  from  X  3500  to 
X  6000  and  the  photo-electric  threshold  for  newly  distilled  Mg 
moved  from  X  3500  to  X  5500  in  twenty-four  hours,  without  any 
apparent  cause. 

In  1913  and  1914,  it  was  urged  by  a  number  of  physicists — 
Fredenhagen  among  others — that  both  the  thermionic  and  photo- 
electric effects  were  essentially  effects  associated  with  the  chemical 
action  between  the  metal  and  an  invisible  film  of  reacting  gas, 
and  that  both  effects  would  disappear  for  metals  completely  de- 
nuded of  gases.  In  support  of  this,  Kustner2  found  that  for  newly 
produced  surfaces  of  zinc  in  a  vacuum  from  which  all  re-acting 

»P.  Z.,  14,  111  (1913). 

*  Ibid.,  15,  68  (1914);  A.  d.  P.,  46,  893  (1915). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


101 


Amoi/nr  of  6a« 


gases  were  most  carefully  excluded,  there  was  no  photo-electric 
effect.  As  he  used  a  quartz  mercury  lamp,  we  may  consider  that 
his  shortest  wave-length  was  X  1849.  A  less  radical  departure 
would  be  to  conclude  that  the  photo-electric  threshold  for  zinc 
had  been  merely  displaced  to  a  value  below  X  1849.  (The  chemical 
action  theory,  so  far  as  thermionics  is  concerned,  was  disproved 
by  O.  W.  Richardson.1)  This  view  received  support  from  some 
experiments  by  Hughes2  on  the  contact  difference  of  potential 
between  platinum  and  newly  distilled  zinc.  Ordinary  clean  zinc 
is  electropositive  to  platinum  by  about  a  volt.  Newly  distilled 
zinc  was  frequently  found  to  be  slightly  electronegative  to  platinum. 
However  the  slightest  trace  of  a  re-acting  gas  served  to  make  it 
strongly  electropositive.  The  elec- 
tropositive character  as  a  function 
of  the  amount  of  gas  admitted  to  the  ^ 
zinc  may  be  represented  diagram-  ^ 
matically  as  in  fig.  5.  The  relations  s 
between  the  photo-electric  threshold  03 
and  the  contact  difference  of  poten- 
tial brought  out  in  the  preceding  sec- 
tion would  then  indicate  that  the 
threshold  for  newly  distilled  zinc  ^ 
should  occur  at  a  shorter  wave-  £ 
length  than  that  for  ordinary  plat- 
inum (X  2800),  but  that  a  trace  of  gas 
would  shift  it  well  towards  the  visible  spectrum.  This  is  precisely 
what  happened  in  Kustner's  photo-electric  experiments.  It  is 
probable  that  the  experimental  conditions  in  Kustner's  work 
allowed  a  more  thorough  removal  of  re-acting  gases  than  in  these 
experiments  on  the  contact  potential,  and  this  would  explain  why 
the  threshold  in  Kustner's  experiments  had  been  displaced  further 
than  might  be  .  anticipated  from  these  experiments.  Kustner's 
experiments  imply  that  the  intrinsic  potential  for  perfectly  gas- 
free  zinc  is  above  6.7  volts,  calculated  from  X  1849  (and  assuming 
that  p2  in  Millikan's  form  of  Einstein's  equation  is  zero,  or  does 
not  enter  into  hv0),  a  value  which,  it  should  be  remarked,  is  appre- 
ciably greater  than  the  thermionic  value  for  platinum  and  tungsten 
obtained  under  conditions  considered  gas  free.  The  whole  ques- 

1  P.  M.,  26,  345  (1913). 

2  Ibid.,  28,  337  (1914). 


FIG.  5. 


102  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

tion  of  contact  differences  of  potential  between  gas-free  metals  may 
well  repay  investigation. 

Experiments  on  the  photo-electric  threshold  of  the  alkali  metals 
have  yielded  conflicting  results.  Wiedmann  and  Hallwachs1 
found  that  the  photo-electric  activity  of  K  disappeared  after 
repeated  distillation  in  the  highest  vacuum.  The  shortest  wave- 
length available  in  their  light  was  probably  X  3300.  Pohl  and 
Pringsheim,2  on  the  other  hand,  found  that  the  photo-electric  effect 
of  K  was  not  appreciably  affected  by  prolonged  boiling,  and,  in 
particular,  that  the  selective  effect  remained  unchanged.  The 
shortest  wave-length  available  was  about  X  4360.  Wiedmann3 
investigated  the  matter  again  and  found  that  the  photo-electric 
effect  of  K  was  reduced  to  less  than  1  per  cent  after  repeated  dis- 
tillations, and,  moreover,  that  the  selective  effect  disappeared 
altogether.  Werner4  found  that  with  sputtered  platinum,  the 
photo-electric  current  increased  considerably  with  the  age  of 
the  surface,  especially  for  the  longer  wave-lengths.  The  photo- 
electric threshold  also  underwent  a  slight  displacement  towards 
longer  wave-lengths.  Elster  and  Geitel5  found  that  boiling  and 
re-distilling  potassium  for  as  long  as  214  hours  had  no  effect  on 
either  the  normal  or  the  selective  effects.  A  similar  result  was 
found  for  cadmium,  the  photo-electric  current  being  substantially 
the  same  whether  the  cadmium  had  been  distilled  seven  times  or 
only  once.  By  investigating  the  effect  with  monochromatic  light 
of  different  frequencies,  Millikan  and  Souder6  found  that  the 
behavior  of  Na  immediately  after  exposing  a  fresh  surface  in  a 
very  good  vacuum  depended  very  largely  on  the  frequency  of  the 
light  used.  Fig.  6  shows  the  sensitivity  to  various  frequencies 
at  successive  time  intervals  after  renewing  the  surface.  Under 
conditions  when  the  absence  of  gases  was  considered  most  com- 
plete, the  Na  was  found  to  be  quite  insensitive  to  X  5461  immediately 
after  exposing  a  new  surface  by  cutting,  but  after  about  two  min- 
utes it  began  to  be  sensitive  and  increased  to  a  maximum  in  about 
twenty  minutes.  Its  behavior  with  regard  to  X  2536  under 
similar  conditions  was  quite  different,  the  sensitivity  at  the  begin- 
ning being  very  high  but  falling  as  the  surface  grew  older.  It  was 

1  V.  d.  D.  P.  G.,  16,  107  (1914). 
*Ibid.,  16,  336  (1914). 

3  Ibid.,  16,  343  (1915). 

4  Ark.  for.  Mat  Astr.  och.  Fysik.,  8,  1  (1913). 

•  P.  Z.,  21,  361  (1920). 

•  P.  R.,  4,  73  (1914);  N.  A.  S.  P.,  2,  19  (1916). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


103 


considered  that  the  X  2536  effect  was  that  of  the  metal  itself, 
while  the  rise  and  fall  of  the  X  5467  effect  corresponded  to  the 
production,  and  subsequent  decay,  of  a  new  substance  formed  at 
the  surface  by  the  reacting  gas.  The  substance  has  the  property 
of  making  the  surface  more  electropositive.  Millikan1  mentions 
that  the  contact  difference  of  potential  between  Na  and  a  CuO 
surface  was  decreased  by  exposing  a  new  Na  surface,  after  which 
it  gradually  rose,  a  result  analogous  to  that  of  Hughes  obtained 
with  freshly  distilled  zinc. 

Much  more  remains  to  be  done  in  establishing  the  exact  nature 


£0  JO  +0 

Time       Mint. 

FIG.  6. 


60 


of  the  causes  giving  rise  to  the  shifts  of  the  photo-electric  threshold. 
The  problem  seems  to  be  intimately  connected  with  that  of  the 
nature  of  the  contact  potential.  Questions  have  been  raised 
from  time  to  time  as  to  whether  a  contact  difference  of  potential 
exists  between  really  pure,  gas-free,  metallic  surfaces,  i.  e.,  whether 
it  is  an  inherent  property  of  the  metal  or  an  incidental  property  of 
the  surface.  Further  information  as  to  this  can  hardly  fail  to  have 
an  important  bearing  on  the  nature  of  photo-electric  thresholds. 
1  P.  R.,  7,  355  (1916). 


104 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


CHAPTER  IV 
TOTAL  PHOTO-ELECTRIC  EFFECT 

PHOTO-ELECTRIC  CURRENT  AND  LIGHT  INTENSITY 

An  important  question  which  has  been  the  subject  of  many 
investigations  since  1913,  as  well  as  before  it,  is  that  of  the  rela- 
tion of  the  magnitude  of  the  photo-electric  current  from  an  il- 
luminated surface  to  the  intensity  of  the  incident  light.  Generally 
speaking,  the  results  have  pointed  to  the  conclusion,  which  one 
would  expect  on  theoretical  grounds,  that  the  photo-electric  cuj> 

rent  is  proportioiiaj  to  the  intensity  of  the  light^ Yet  some  careful 

investigations    indicated    a    departure    from    the    proportionality 


FIG.  6a. 


FIG.  Qb. 


law.  In  view  of  important  practical  applications  of  the  law  to 
the  measurement  of  light  intensity,  many  researches  have  been 
carried  out  to  test  its  validity. 

Investigations  have  been  made  chiefly  on  the  alkali  metals 
inasmuch  as  these  are  sensitive  to  visible  light.  The  photo-electric 
cell  consists  essentially  of  a  surface  to  be  illuminated,  forming 
one  electrode,  and  another  electrode,  whereby  an  accelerating 
potential  can  be  applied  to  drive  the  photo-electrons  across  the 
cell.  Two  extreme  types  are  shown  in  the  diagrams  (figs.  6a,  66). 
The  pure  photo-electric  effect  is  that  obtained  in  the  absence  of  any 
gas.  However,  to  use  the  magnification  (which  may  conveniently 
amount  to  a  100-fold  or  more)  of  the  photo-electric  current  by  ioniza- 
tion  by  collision,  it  is  customary  to  fill  the  cell  with  gas  at  a  pres- 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  105 

sure  of  about  1  mm.  To  avoid  reactions  between  the  sensitive 
surface  and  the  gas,  the  gas  is  usually  He,  A,  or  Ne.  By  passing 
a  glow  discharge  through  the  cell  when  it  contains  hydrogen,  a 
hydride  of  the  alkali  metal  is  formed  which  is  much  more  sensitive 
even  than  the  pure  metal.  In  such  cells  it  is  the  practice  to  replace 
the  hydrogen  after  the  new  sensitive  surface  has  been  formed  by 
an  inert  gas. 

Now,  in  testing  the  proportionality  law,  the  fundamental  thing 
to  be  investigated  is  the  ratio  of  the  number  of  photo-electrons 
liberated  at  the  surface  to  the  intensity  of  the  light.  In  the  absence 
of  any  gas,  it  is  evident  that  saturation  must  be  obtained,  while, 
in  the  presence  of  a  gas,  the  multiplying  factor  due  to  ionization 
by  collision  must  not  vary. 

An  extensive  series  of  experiments  was  made  by  Ives1  and  also 
by  him  in  conjunction  with  Dushman  and  Karrer.2  In  the  earlier 
of  these  papers,  results  were  given  indicating  that  the  ratio  of  the 
current  to  the  intensity  of  the  light  was  not  constant,  but  was  a 
complicated  function  of  the  intensity,  applied  potential,  illuminated 
surface,  and  gas.  The  cells  usually  contained  potassium  treated 
by  a  glow  discharge  in  hydrogen,  and  filled  with  an  inert  gas  at  a 
low  pressure  to  magnify  the  effect.  Finally,  the  source  of  the 
departure  from  the  linear  relation  was  traced  down  to  the  presence 
of  insulating  surfaces  in  the  cells  as  usually  constructed.  These 
surfaces  generally  acquire  charges  from  photo-electrons,  or  ions, 
hitting  them,  and  these  charged  surfaces,  in  turn,  distort  the  elec- 
tric field  which  affects  the  flow  of  electricity  across  the  cell.  In 
view  of  the  fact  that  these  insulating  surfaces  (which  may  not  be 
perfect  insulators)  may  acquire  different  surface  charges  under 
different  conditions  of  illumination,  field  intensity,  etc.,  it  is  little 
wonder  that  the  effect  could  be  otherwise  than  an  incalculable 
modification  of  the  strict  proportionality.  The  final  result  of 
this  series  of  investigations  is  that  a  strictly  linear  relation  can 
be  obtained  between  illumination  and  photo-electric  current,  pro- 
vided close  attention  is  paid  to  the  design  of  the  cell.  They  point 
out  that  the  design  of  the  cell  due  to  Hughes,3  in  which  the  inside 
of  a  bulb  is  almost  completely  covered  with  distilled  sodium 
and  the  other  electrode  is  a  brass  rod  (as  shown  in  fig.  66  above), 
so  that  there  are  no  exposed  insulating  surfaces  anywhere  near 

1  A.  P.  J.,  39,  428  (1914);  40,  182  (1914);  46,  241  (1917). 

2  Ibid.,  43,  9  (1916). 

'  P.  M.,  35,  679  (1913). 


106  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

where  the  current  flows,  satisfies  the  necessary  conditions.  Ives 
found  it  impossible  to  make  cells  identical  in  properties.  The 
curve  of  sensitivity  with  wave-lengths  varied  considerably  from 
one  cell  to  another  prepared  under  identical  conditions.  Kunz1 
found  that  when  the  potential,  pressure  and  light  intensity  were 
so  adjusted  that  a  visible  glow  was  to  be  seen  in  the  photo-electric 
cell,  the  current  was  not  proportional  to  the  light  intensity.  Evi- 
dently, the  magnification  due  to  ionization  by  collision  is  no  longer 
a  constant  under  these  conditions. 

Elster  and  Geitel2  discussed  the  various  ways  whereby  faulty 
design  of  a  photo-electric  cell  causes  an  apparent  departure  from 
the  proportionality  law.  When  the  possibility  of  charges  on  the 
glass  wall  was  avoided,  they  found  strict  proportionality  from 
30,000  lux  0/3  of  sunlight)  down  to  6  X  10 ~4  lux.  (A  lux  =  1 
candle  metre.)  The  lowest  illumination  with  a  potential  of  200 
volts  across  the  cell  was  2.4  X  10 ~6  lux  and  gave  a  current  of 
1.8  X  10 ~12  amp.  In  a  second  paper3  they  found  an  apparent 
departure  from  the  proportionality  law  with  strong  light,  but 
this  was  traced  down  to  a  discontinuity  in  the  magnification  of  the 
photo-electric  current  by  ionization  by  collision.  In  this  region, 
they  state  that  if  the  cell  is  used  completely  evacuated,  the  strict 
proportionality  is  observed.  In  a  later  paper4  they  found  that 
the  proportionality  law  held  down  to  light  as  feeble  as  3  X  10  ~9 
erg  per  sq.  cm.  per  sec.  for  blue  light  and  2  X  10 ~7  erg.  per  sq.  cm. 
per  sec.  for  orange  light,  the  former  value  being  below  the  "thres- 
hold" value  for  the  human  eye. 

Kunz  and  Stebbins5  found  that  the  proportionality  law  held 
exactly  for  a  rubidium  cell  containing  neon.  Kunz6  found  a  slight 
departure  from  the  proportionality  law  with  high  intensities. 
With  the  older  type  of  spherical  cell  with  illuminated  central 
electrode,  there  was  a  marked  departure  from  the  proportionality 
law,  while  a  cell  in  which  the  two  electrodes  were  parallel  plates 
gave  the  best  agreement  with  the  law.  Proportionality  was  found 
to  hold  over  a  range  from  1  to  1300. 

A  consideration  of  these  results,  taken  altogether,  seems  to 
point  beyond  all  doubt  to  the  result  that  there  is  a  strict  propor- 

P.  R.,  13,  310  (1919). 

P.  Z.,  14,  741  (1913). 

Ibid..  15,  610  (1914). 

P.  Z.,  17,  268  (1916). 

P.  R.,  7,  62  (1916). 
•A.  P.  7.,  45,  69  (1917);  P.  R.,  9,  175(1917). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  107 

tionality  between  intensity  of  illumination  and  the  number  of 
photo-electrons  emitted,  and  that  this  holds  over  a  wide  range 
extending  from  illumination  too  feeble  to  be  visible,  to  illumination 
comparable  with  sunlight.  Departures  from  the  law  appear  to 
be  due  to  faulty  design  in  the  cell  which  permits  spurious  effects, 
or  else  to  the  alteration  in  the  multiplying  factor  of  ionization  by 
collision  under  extreme  conditions. 

PHOTO-ELECTRIC   PHOTOMETRY 

The  numerous  articles  on  the  applications  of  the  photo-electric 
cell  to  the  measurement  of  light  intensities  show  how  valuable 
such  cells  have  been  found  for  this  purpose.  They  are  of  particular 
value  under  conditions  where  thermopiles  are  unsatisfactory  and 
difficult  to  work  with,  as,  for  example,  with  weak  visible  light  or 
in  the  ultraviolet.  For  satisfactory  work  as  measurers  of  light 
intensity,  it  is,  of  course,  necessary  to  ensure  that  the  design  of  the 
cell  is  such  that  there  is  no  departure  from  strict  proportionality 
between  the  light  intensity  and  current.  For  applications  of  the 
kind  contemplated  here,  it  is  well  to  arrange  for  as  high  a  degree 
of  sensitivity  as  one  can  get.  First,  arrange  to  use  as  much  of  the 
incident  light  as  possible.  When  light  falls  upon  a  clean  sodium 
surface,  probably  over  99  per  cent  is  reflected  and  so,  in  a  cell  of 
the  first  type  in  fig.  6a,  is  lost  so  far  as  the  photo-electric  effect  is 
concerned.  By  means  of  a  design  of  cell  suggested  by  "black 
body  enclosures"  as  used  in  the  study  of  heat  radiation,  such  as 
the  cell  described  by  Hughes1  (fig.  66)  it  is  possible  to  trap  almost 
all  the  light  and  so  make  it  effective  photo-electrically.  Such  a 
cell  is  prepared  by  distilling  an  alkali  metal  on  to  the  inside  of  a 
suitably  shaped  bulb,  and  arranging  to  leave  a  small  aperture 
just  big  enough  for  the  light  to  enter.  Light  which  enters  is  re- 
flected to  and  fro  inside  and  each  time  contributes  to  the  photo- 
electric effect.  To  increase  the  sensitivity  further,  it  is  customary 
to  pass  a  glow  discharge  through  hydrogen  in  the  cell  to  convert 
the  metal  into  the  more  sensitive  hydride.  Finally,  one  makes 
use  of  the  magnification  of  the  original  photo-electric  current  from 
the  surface,  by  filling  the  cell  with  an  inert  gas,  such  as  argon, 
and  choosing  the  most  favorable  accelerating  potential  to  give  the 
most  convenient  magnification. 

One  disadvantage  of  photo-electric  cells  is  that  no  two  cells 
have  exactly  the  same  sensitivity  for  different  wave-lengths.  So, 

1  P.  M,,  35,679  (1913) 


108  REPORT  ON  PHOTO-ELECTPICITY:  A.  LL.  HUGHES 

when  one  uses  photo-electric  cells  to  compare  energies  in  different 
parts  of  the  spectrum,  it  is  necessary  to  standardize  each  cell, 
either  by  a  thermopile  (using  intense  enough  light,  of  course,  to 
affect  it)  or  by  using  a  source  of  light  in  which  the  distribution  of 
energy  is  known.  For  studying  variations  in  monochromatic 
light,  no  such  calibration  is  necessary. 

As  we  have  seen,  a  cell  of  the  type  designed  by  Elster  and  Geitel 
is  more  sensitive  than  the  human  eye.  Such  a  cell  so  arranged 
that  its  effect  is  strongly  magnified  by  the  use  of  three  electrode 
valves,  should  open  up  possibilities  of  a  big  field  of  quantitative 
measurements  in  regions  hitherto  unexplored  (e.  g.,  weak  phos- 
phorescence, scattering  of  light,  resonance  emission,  etc.). 

A  method  of  magnifying  the  effect  of  a  photo-electric  cell  by  a 
three  electrode  valve  has  been  described  by  Kunz,1  and  by  Pike.2 
Pike  obtained  amplifications  as  large  as  15,000.  Meyer,  Rosen- 
berg and  Tank3  obtained  amplifications  up  to  about  15,000.  In 
later  experiments  values  up  to  125,000  were  obtained.  They 
found  that  the  amplification  was  quite  constant  for  small  photo- 
electric currents. 

The  photo-electric  cell  is  well  adapted  for  measuring  stellar 
magnitudes,  fluctuations  of  star  light,  and  corona  intensities 
during  eclipses.  It  will  be  remembered  that  unless  spectral 
resolution  be  resorted  to,  the  sensitivities  of  photo-electric  cells 
differ  from  each  other  and  from  that  of  the  human  eye. 
Just  as  the  eye  is  most  sensitive  to  yellow-green  light,  so  cells 
containing  Cs,  Rb,  K,  Na,  Cd  and  Zn  have  their  own  proper 
maxima  running  from  yellow  far  into  the  ultraviolet,  in  the  order 
indicated.  Miss  Seiler4  found  that  the  maxima  for  photo-electric 
cells  were  at  X  4050,  X  4200,  X  4410,  X  4730,  and  X  5390  when  the 
surfaces  were  Li,  Na,  K,  Rb,  and  Cs.  In  every  case,  sensitizing 
the  surface  by  passing  a  glow  discharge  through  hydrogen  over 
the  alkali  metal  caused  a  shift  in  the  maximum  towards  the  longer 
wave-lengths. 

Elster  and  Geitel5  describe  photometers  with  Cd  or  Zn  as  the 
sensitive  surface,  specially  suitable  for  measurements  on  the  ultra- 
violet radiation  of  the  sun  and  stars.  Guthnick6  used  a  photo- 

1  P.  R.,  10,  205  (1917). 

2  Ibid.,  13,  102  (1919). 

«  Archeves  des  Sciences,  2,  260  (1920). 

*  P.  R.,  15,  550  (1920). 

5  P.  Z.,  15,  1  (1914);  16,  405  (1915). 

•  Nature,  103,  53  (1919);  Astr.,  Nachr.,  No  4976. 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES          109 

electric  cell  to  determine  the  amount  of  light  reflected  by  the 
planets  from  the  sun  at  different  parts  of  their  orbit.  A.  F.  and 
F.  A.  Lindemann1  discuss  the  various  applications  of  photo-electric 
cells  to  astronomy.  Among  other  applications  may  be  mentioned 
the  study  of  the  light  reflected  by  the  planets  for  evidence  as  to 
fluctuations  in  the  light  of  the  sun.  They  also  point  out  the  use- 
fulness of  photo-electric  cells  in  the  study  of  the  light  from  diffused 
luminous  areas  such  as  nebulae  and  comets. 

Kunz  and  Stebbins2  gave  details  of  the  use  of  photo-electric 
cells  for  studying  the  light  from  the  corona  during  the  eclipse  of 
June  8,  1918.  Stebbins  and  Dershem3  used  potassium  cells  for 
studying  the  fluctuations  of  the  light  from  Nova  Aquilae,  No.  3. 
Hulburt4  used  a  photo-electric  cell  successfully  for  investigating 
the  reflecting  power  of  metals  in  the  ultraviolet  region.  Nathan- 
son5  investigated  the  reflecting  power  of  the  alkali  metals  by  a 
photo-electric  method. 

A  recent  paper  by  Elster  and  Geitel6  deals  with  an  apparent 
continuation  of  the  photo-electric  effect  for  potassium  for  a  short 
time  after  the  light  is  cut  off.  This  was  found  to  be  due  to  a  weak 
phosphorescence  in  the  photo-electric  cell.  This  effect  may  intro- 
duce an  error  in  photo-electric  photometry  when  light  intensities 
are  changing  rapidly. 

It  is  convenient  here  to  give  a  short  account  of  the  discovery 
of  a  new  and  exceedingly  interesting  kind  of  photo-electric  effect, 
which  was  discovered  by  Mr.  Shenstone  in  the  Physics  Laboratory 
of  Princeton  University.  The  photo-electric  current  from  a 
bismuth  plate  was  found  to  depend  very  much  on  the  magnitude 
of  a  current  passing  through  the  plate.  A  thin  square  plate  of 
bismuth  was  mounted  in  a  vacuum  so  that  an  electric  current 
could  be  passed  across  it,  i.  e.,  from  one  side  of  the  square  to  the 
opposite  side.  The  photo-electric  current  from  the  bismuth, 
illuminated  by  light  from  a  mercury  arc,  was  found  to  be  inde- 
pendent of  the  current  through  the  bismuth,  as  long  as  it  was 
below  1.1  amperes.  Any  increase  beyond  this  up  to  3  amperes 

1  Roy.  Astr.  Soc.  Monthly  Notices,  79,  343  (1919). 

2  A.  P.  y.,49,  137  (1919). 

3  Ibid.,  49,  343  (1919). 

4  Ibid.,  41,  400  (1915);  46,  I  (1917). 

5  Ibid.,  44,  137  (1916). 

6  P.  Z.,  21,  361  (1920). 


110  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

or  more  caused  an  increase  in  the  photo-electric  current,  a  change 
amounting  to  as  much  as  60  per  cent  being  observed  as  the  curr  ent 
was  altered  from  1  to  3  amperes.  The  effects  showed  considerable 
time  lag,  thus  the  equilibrium  value  of  the  photo-electric  current 
was  not  obtained  until  more  than  an  hour  had  elapsed  after  the 
current  through  the  bismuth  had  been  maintained  at  a  steady 
value.  Control  experiments  showed  that  the  heat  effect  and  the 
magnetic  effect  of  the  current  were  not  responsible  for  the  increase 
in  the  photo-electric  current.  (The  effects  persisted  after  the  cur- 
rent through  the  bismuth  was  stopped,  fatigue,  however,  setting  in 
and  reducing  it.)  A  similar,  but  much  smaller,  effect  was  obtained 
with  zinc. 


CHAPTER  V 

THE   PHOTO-ELECTRIC   EFFECT  AS  A   FUNCTION   OF  THE 

FREQUENCY  AND  STATE  OF  POLARIZATION  OF  THE 

LIGHT 

RICHARDSON'S  STATISTICAL  THEORY 

A  number  of  investigations  on  the  photo-electric  effect  as  a 
function  of  the  frequency  of  the  light  have  been  carried  out.  The 
only  theoretical  expressions  for  this  function  are  those  due  to 
O.  W.  Richardson.1  They  are  deduced  along  the  following  lines. 
Consider  a  cavity  in  a  piece  of  matter,  containing  an  atmosphere 
of  electrons  in  equilibrium,  so  that  just  as  many  enter  the  matter 
through  the  surface  as  leave  it.  Richardson  finds,  by  applying 
the  usual  methods  of  the  kinetic  theory  of  gases,  that  the  number 
of  electrons  impinging  on  a  unit  area  of  the  surface  from  the  atmos- 
phere is 


r  (1) 

where  A  is  a  constant,  T  the  temperature,  R  the  gas  constant  for  a 
single  molecule,  and  w0  the  work  done  by  the  electron  against 
the  forces  tending  to  retain  it  in  the  matter.  The  number  of 
electrons  leaving  the  surface,  on  the  other  hand,  is  assumed  to  be 
determined  by  the  total  radiation  (acting  photo-electrically) 
which  is  in  equilibrium  with  the  matter  at  that  temperature  and, 
therefore,  traversing  it  in  all  directions.  The  energy  density  of 
the  radiation  between  the  frequencies  v  and  v  +  dv  is 
>  P.  M.,  23,  594  (1912);  24,  574  (1912);  27,  476  (1914). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  111 

E(v,  T)<fr 

where  E(»s  T)  is  some  function  of  v  and  T  (such  as  Planck's  or 
Wien's).  The  rate  of  absorption  of  this  energy  is 

-  cEO/,  T)dv 
4 

where  c  is  the  velocity  of  light  and  e  the  emissivity.  Let  F(?) 
be  the  number  of  electrons  emitted  per  unit  area  of  the  surface 
when  radiation  of  frequency  v  passes  through  it.  Consequently, 
the  total  number  of  electrons  emitted  in  unit  time  is 


N2  =  -    f  eFOOEds  T)<fo  (2) 

4-  •/ 


4 

o 

In  his  earlier  papers  Richardson  used  Wien's  formula 

E(v,  T)  =  ^  hv3e~** 
while  in  the  last  paper  Planck's  formula  was  used 


e  RT—  1 

When  there  is  equilibrium,  the  value  of  NI  (equation  (1))  must 
equal  N2  (equation  (2)).     The  equations  to  be  satisfied  are 

_hv  _v>o 

(v)hv3e  RTdv  =  AT20  RT  (3) 


or 


c.  §5   f  tf  W  _ 
4  - 


(4) 


*»-! 

according   to  whether  Wien's  or  Planck's   expression  is  adopted. 
A  solution  of  (3)  is 
€FV    =  0 


,  when  w0<hv<  °o  (5) 

hv 

w0  =  hv0 ,  where  v0  is  the  frequency  of  the  photo-electric  threshold. 
Should  (4)  be  considered  instead  of  (3)  the  value  of  the  function 
F  (v)  above  the  photo-electric  threshold  takes  a  slightly  different 
form,  viz., 


112 


REPORT    ON    PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


Returning  to  the  solution  given  in  equation  (5)  it  will  be  seen  that 

Q 

eFO)  has  a  maximum  value,  which  occurs  when  v  =  -    v0.     This 
can  be  tested  by  experiment. 

EXPERIMENTAL  RESULTS 

Richardson  points  out  that  the  photo-electric  experiments  are 
carried  out  under  conditions  different  from  those  stipulated  in  the 
theoretical  discussion.  Experimentally  we  have  to  deal  with  the 
rate  of  emission  of  photo-electrons  under  given  illumination  and 
not  with  a  state  of  equilibrium  as  was  assumed  in  the  theoretical 
discussion  when  the  matter  was  supposed  to  be  traversed  by  iso- 


AL 

Ty^ 

/ 

" 

f 

\ 

-< 

80 

/ 

f 

\ 

/ 

/ 

t>v 
40 
20 
0. 

/ 

1 

/ 

\ 

/ 

\ 

7 

v    80    90    100    no     lao   /jo    140  /so 
Frequency 

FIG.  7. 

tropic  radiation.  Also,  the  functions  given  above  are  solutions 
of  the  equations,  but  not  necessarily  complete  solutions.  The 
solutions  imply  that  the  function  has  the  same  shape  for  all  sub- 
stances, the  nature  of  the  substance  only  enters  by  fixing  v0,  the 
frequency  of  the  photo-electric  threshold. 

Compton  and  Richardson1  investigated  the  photo-electric  sen- 
sitivity of  Pt,  Al,  Na  and  Cs  with  monochromatic  light  of  different 
frequencies  to  test  the  expression  given  in  (5).  The  results  for 
Al  and  Na  are  shown  in  the  accompanying  figures.  For  both 
Al  and  Na,  there  is  a  definite  maximum  sensitivity  which,  however, 
is  much  more  sharply  defined  than  the  theoretical  maximum. 
Its  position  is  in  accordance  with  the  predictions  of  the  theory. 

1  P.  M.,  26,  549  (1913). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


113 


It  will  be  noticed  that  Na  has  two  well  defined  maxima.  The 
shape  of  the  experimental  curves  differs  considerably  from  that 
given  by  the  theory.  The  available  ultraviolet  region  is  so  limited 
that  no  second  maximum  could  be  looked  for  with  Al,  nor  could 
the  curve  be  carried  as  far  as  the  first  maximum  for  Pt.  In  a 
later  paper,  Richardson  and  Rogers1  reduce  these  experimental 
results  to  absolute  values  which  are  expressed  in  coulombs  per 
calorie. 

^  Richardson2  went  into  the  question  of  whether  it  was  possible 
to  explain  thermionic  emission  of  electrons  by  a  photo-electric 
emission  due  to  the  radiation  from  the  hot  body  illuminating 
80 


V50     60     70     80     90    100    110    /^0  /30  14-0  150 

Frequency 


FIG.  8. 


itself  (an  "autophoto-electric"  effect).  He  found,  on  thermodynam- 
ical  assumptions,  that  the  photo-electric  current  from  a  sub- 
stance when  illuminated  by  full  radiation  characteristic  of  a  tem- 
perature, T,  should  have  the  form 


This  equation  is  identical  with  that  for  the  ordinary  thermionic 
emission.  W.  Wilson3  found  that  the  photo-electric  current  from 
sodium  potassium  alloy  obeyed  this  law  as  the  temperature  T 

1  P.  M.,  29,  618  (1915). 

2  "Emission  of  Electricity  from  Hot  Bodies,"  p.  101. 

3  P.  R.  S.,  93,  359  (1917). 


1L4 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


of  the  source  of  light  was  varied.  (The  source  was  arranged 
so  as  to  give  approximately  black  body  radiation.)  Richardson 
concludes,  however,  that  though  the  photo-electric  current  de- 
pends in  the  same  way  on  the  temperature  as  the  thermionic 
emission,  the  photo-electric  current  forms  at  the  best  but  a  very 
insignificant  fraction  of  the  thermionic  current.  Langmuir1  men- 
tions that  the  magnitude  of  the  photo-electric  current  produced 
by  radiation  from  an  incandescent  filament  was  only  one-mil- 
lionth part  of  the  thermionic  current  from  the  filament.  A  de- 
posit of  tungsten  was  formed  on  the  inside  of  a  bulb  by  evaporation 
from  a  tungsten  filament  at  the  center,  and  a  photo-electric  current 


a+oo 


from  the  deposit  was  obtained  on  making  the  filament  incandes- 
cent. 

Souder2  carried  out  a  number  of  investigations  on  K,  Na  and  Li. 
In  Compton  and  Richardson's  experiments,  the  Na  surface  was 
prepared  by  distillation,  while  in  his  experiments  the  surface  was 
produced  by  cutting  in  vacuo.  The  results  are  shown  in  fig.  9. 
It  will  be  noticed  that  there  is  no  evidence  of  a  second  maximum 
in  Souder's  experiments,  as  would  be  expected  from  Compton 
and  Richardson's  experiments. 

In  any  attempt  to  build  up  a  theoretical  formula  for  the  photo- 
electric sensitivity  of  a  substance,  it  will  be  seen  that  any  effort 
to  check  it  accurately  by  experiment  will  be  subject  to  serious 
difficulties.  The  number  of  electrons  which  emerge  from  a  surface 
in  any  photo-electric  experiment  will  be  a  complicated  function 

*J.A.  C.  S.,  42,  2190(1920). 
1  P.  R..  8.  310  U816). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


115 


of  the  distance  to  which  the  light  penetrates  into  the  surface  and 
of  the  absorption  coefficient  of  the  photo-electrons  by  the  sub- 
stance. Hence,  the  photo-electric  sensitivity,  as  obtained  in  the 
experiments  quoted,  is  subject  to  unknown  corrections  before 
what  may  be  called  the  true  photo-electric  sensitivity  can  be  ob- 
tained. In  this  connection,  reference  may  be  made  to  a  paper 
by  O.  W.  Richardson.1 

NORMAL  AND  SELECTIVE  EFFECTS 

One  of  the  most  puzzling  phenomena  in  the  photo-electric  effect 
is  the  so-called  "selective  photo-electric  effect,"  which  appears 
in  some  cases.  According  to  the  careful  experiments  of  PohP 
the  photo-electric  current  per  unit  of  energy  absorbed  from  the 
light  is,  in  general,  independent  of  the  angle  of  incidence  and  also 
of  the  state  of  polarization  of  the  incident  beam.  In  certain  cases 
(the  alkali  metals),  however,  there  is  a  remarkable  departure 
from  this  rule,  the  photo-electric  current  being  many  times  stronger 
when  the  electric  vector  in  the  light  beam  has  a  component  per- 
pendicular to  the  surface  than  when  it  is  wholly  parallel  to  the 
surface.  This  effect,  the  "selec- 
tive" effect,  is  restricted  to  a  range  / 
of  wave-lengths  characteristic  of 
the  metal,  while  the  "normal" 
effect  has  no  such  restriction.  A 
typical  diagram  of  the  effect  is 
shown  in  fig.  10,  where  E||  and 
E_l_  denote  the  relation  of  the 
electric  vector  to  the  plane  of  in- 
cidence. Pohl  and  Pringsheim 
have  obtained  a  large  mass  of  ex- 
perimental data  as  to  these  effects, 

which  is  summarized  in  the  writer's  "Photo-Electricity."  No 
satisfactory  explanation  has  yet  been  given  of  the  phenomena. 

With  unpolarized  light,  the  two  effects  are  superposed  showing 
a  well-marked  maximum  coinciding  with  the  maximum  of  the 
selective  effect.  In  the  earlier  investigations,  it  was  generally 
assumed  that  a  pronounced  maximum  in  the  photo-electric  current 
curve,  when  the  substance  was  illuminated  by  light  of  different 
wave-lengths,  indicated  a  selective  effect.  While  this  is  no  doubt 

1  P.  M.,  31,  149  (1916). 

*  V.  d.  D.  P.  G.,  10,  339,  609,  715  (1909). 


116  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

true  in  the  case  of  the  alkali  metals,  many  recent  investigations 
show  a  maximum  in  the  photo-electric  current  curves  plotted 
against  wave-lengths  when  a  selective  effect  does  not  exist.  The 
theoretical  investigations  of  Richardson,  already  referred  to, 
point  to  a  maximum  photo-electric  current  at  a  certain  wave- 
length, without  any  reference  to  the  state  of  polarization  of  the 
light,  or  to  the  inclination  of  the  beam.  It  is,  therefore,  advisable 
to  restrict  the  term  "selective  effect"  to  the  narrow  limits  originally 
suggested  by  Pohl  and  Pringsheim.  It  should  be  kept  for  that 
occasion  when,  reckoned  for  equal  amounts  of  absorbed  light,  the 
photo-electric  sensitivity  is  abnormally  large  when  the  electric 
vector  in  the  light  beam  has  a  component  perpendicular  to  the 
surface,  as  compared  to  the  case  when  it  is  entirely  parallel  to  it. 
A  criterion  equivalent  to  this,  which  in  some  cases  may  be  more 
convenient  to  employ,  is  an  accentuation  of  the  maximum  as  the 
angle  of  incidence  is  increased.  An  illustration  of  this  may  be 
taken  from  Pohl  and  Pringsheim's  work.  Na-K  alloy  and  Ca  both 
show  a  maximum  (which  is,  however,  much  less  pronounced  in  the 
case  of  Ca)  in  the  photo-electric  current  curves  when  unpolarized 
light  is  used.  However,  as  the  angle  of  incidence  is  increased, 
the  maximum  becomes  more  and  more  pronounced  in  the  case 
of  Na-K  alloy,  but  in  the  case  of  Ca,  it  becomes  less  and  finally 
vanishes.  A  direct  test  with  polarized  light  shows  that  Ca  does 
not  possess  a  selective  effect,  while  Na-K  alloy  does.  It  is  evident, 
then,  that  something  more  than  the  mere  presence  of  a  maximum 
in  the  photo-electric  current  curves  when  plotted  against  the 
wave-length,  is  necessary  to  determine  whether  a  real  selective 
effect  is  present.  It  may  be  remarked  that  Pohl  and  Pringsheim 
assumed  that  Li  had  a  selective  effect  because  of  the  presence  of  a 
maximum  at  X  2800,  when  illuminated  by  unpolarized  light,  on 
the  analogy  of  the  maxima  obtained  for  the  other  alkali  metals, 
for  most  of  which  the  existence  of  the  selective  effect  had  been 
directly  demonstrated  with  polarized  light.  Had  they  worked 
on  Li,  after  discovering  that  the  maximum  in  the  case  of  Ca  did 
not  necessarily  imply  a  selective  effect,  they  would  no  doubt  have 
made  further  tests  using  polarized  light. 

Millikan  and  Souder1  investigated  the  photo-electric  current 
from  a  sodium  surface  as  a  function  of  the  wave-length  of  light 
using  perpendicular  incidence.  They  found  a  maximum  sen- 

1  N.  A.  5.  P.,  2,  19  (1916);  P.  R.,  8,  310  (1916). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  117 

sitivity  at  the  same  wave-length  as  that  at  which  Pohl  and  Pring- 
sheim  located  the  maximum  of  the  selective  effect.  They  also 
found  that  the  magnitude  of  the  maximum  depended  very  much 
upon  the  age  of  the  surface,  indeed,  for  very  new  surfaces  the 
maximum  could  scarcely  be  detected.  They  are  inclined  to  regard 
the  selective  effect  as  merely  the  normal  effect  in  the  neighborhood 
of  an  absorption  band,  i.  e.,  where  one  particular  frequency  pre- 
dominates. 

The  maximum  velocities  of  the  electrons  in  the  region  where 
the  selective  effect  is  most  marked  fall  in  with  Einstein's  quantum 
relation  exactly  as  they  do  in  regions  where  only  the  normal  effect 
exists,  a  result  verified  by  Millikan's  accurate  experiments  on  Na 
and  Li  in  the  determination  of  "h." 

Hughes1  found  that  there  was  a  small  difference  in  the  distribu- 
tion of  the  electrons  emitted  in  the  selective  effect  as  compared 
with  that  of  those  emitted  in  the  normal  effect.  This  difference, 
however,  was  quite  small  compared  with  the  difference  between 
the  total  currents  for  the  two  effects.  The  difference  in  the  dis- 
tribution could  be  attributed  to  a  difference  in  the  direction  dis- 
tribution, or  to  a  difference  in  the  velocity  distribution  of  the  photo- 
electrons,  that  is,  the  effects  could  be  explained  by  assuming  that 
in  the  selective  effect  the  photo-electrons  were  somewhat  more 
crowded  towards  the  perpendicular  to  the  surface,  or  that  they 
were,  on  the  whole,  slower  than  in  the  normal  effect.  In  a  later 
paper2  it  was  shown  that  a  difference  in  the  direction  distribution 
existed,  the  photo-electrons  in  the  selective  effect  being  emitted, 
on  the  whole,  in  directions  nearer  to  the  perpendicular  to  the 
surface  than  in  the  normal  effect. 

If,  over  the  region  in  which  the  selective  effect  existed,  one 
found  that  the  light  polarized  in  the  E||  plane  were  absorbed  in  a 
much  thinner  layer  of  the  surface  than  light  polarized  in  the  E_]_ 
plane,  one  would  have  a  natural  explanation  for  the  existence  of 
the  selective  effect.  According  to  Pohl  and  Pringsheim3  the 
position  of  the  selective  effect  is  that  in  which  the  reflecting  power 
is  exceptionally  high,  and  high  reflecting  power,  on  optical  theory, 
goes  with  rapid  absorption.  They  did  not  investigate  whether 
the  reflecting  power  in  the  region  of  the  selective  effect  differed 
markedly  for  beams  polarized  in  the  two  principal  planes.  Re- 

'  P.  M.,  31,  100  (1916). 

2  P.  R.,  10,  490  (1917). 

3  V.  d.  D.  P.  G.,  15,  173  (1913). 


118 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


cently,  some  experiments  on  this  point  have  been  carried  on  by 
Miss  Mabel  Frehafer,1  who  investigated  the  reflection  and  ab- 
sorption of  light  by  K  and  Na.  The  ratios  of  the  reflecting  powers 
of  Na  and  K  surfaces  for  light  polarized  in  the  two  principal  planes 
are  shown  in  fig.  11.  It  will  be  seen  that  for  both  Na  and  K,  a 
remarkable  variation  in  the  ratio  takes  place  in  the  ultraviolet. 
For  Na,  the  minimum  in  the  curves  is  approximately  near  the 
position  of  the  selective  maximum.  Any  association  between 
the  two  is  made  doubtful,  however,  by  the  fact  that  no  such  asso- 
ciation appears  for  K.  Experiments  were  carried  out  on  the 


REFZ.ECT/O/V  OF  POLARIZIO  LIGHT 


LSOO      3000      3600     4-000      4500      SOOO 


FIG.  11. 


transparency  of  thin  films  of  K  and  Na,  but  no  specially  marked 
effects  were  noted  in  the  selective  region . 

Should  further  experiments  show  definitely  that  no  explanation 
of  the  selective  effect  in  terms  of  unequal  absorption  of  the  beams 
polarized  in  the  two  principal  planes  is  possible,  it  will  be  necessary 
to  look  elsewhere  for  an  explanation.  Possibly  there  are  systems 
in  the  alkali  metals  so  orientated  that  they  are  more  easily  broken 
up  by  light  which  has  a  component  of  electric  force  perpendicular 
to  the  surface  than  when  the  electric  force  is  entirely  parallel  to  it. 
So  far  as  energy  relations  go,  the  selective  effect  appears  to  be 
governed  by  the  quantum  relation,  just  as  is  the  normal  effect. 

1  P.  R.t  15,  110  (1920). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  119 

CHAPTER  VI 
PHOTO-ELECTRIC  PROPERTIES  OF  THIN  FILMS 

Certain  investigations  recorded  in  the  writer's  "Photo-Electricity" 
implied  that  the  photo-electrons  from  thin  films  had  necessarily 
high  emission  velocities.  Stuhlmann  and  Compton1  found  that 
there  was  no  departure  from  Einstein's  law,  provided  adequate 
care  was  taken  to  prevent  the  formation  of  charged  layers,  ap- 
parently due  to  thin  films  of  oil  or  grease  forming  on  the  surfaces 
investigated.  They  also  found  that  the  photo-electric  effect  of 
sputtered  platinum  was  greater  than  that  of  ordinary  platinum. 

Stuhlmann2  took  up  the  question  of  the  maximum  velocities 
of  photo-electrons  emitted  from  thin  films  of  Pt  on  quartz  plates. 
He  confirmed  Robinson's  result  that  the  maximum  emission  energy 
of  the  photo-electrons  was  about  40  per  cent  greater  when  they 
left  the  emergence  side  than  when  they  left  the  incidence  side.  At 
the  time  these  experiments  were  carried  out,  the  investigations 
of  the  emission  energies  of  photo-electrons  from  ordinary  surfaces 
(i.  e.,  from  the  incidence  side)  all  led  to  values  of  "h"  about  10  per 
cent  to  20  per  cent  too  low,  and  it  was  suggested  therefore,  as  a 
result  of  these  velocity  measurements  on  thin  films,  that  the  correct 
value  for  "h"  might  be  obtained,  provided  that  the  velocities  were 
measured  for  the  emergence  side.  In  view  of  Millikan's  proof 
that  "h"  can  be  obtained  accurately  from  experiments  on  the 
velocities  of  photo-electrons  from  the  incidence  side,  this  explana- 
tion falls  to  the  ground,  and  indeed  renders  it  exceedingly  doubtful 
whether  the  photo-electrons  can  possibly  have  a  greater  maximum 
energy  on  the  emergence  side.  The  differences  in  the  velocities 
are  probably  spurious,  though  it  is  difficult  to  say  where  the  experi- 
mental arrangements  are  faulty. 

Robinson3  measured  the  photo-electric  effect  per  unit  light 
energy  absorbed  for  thin  films  as  a  function  of  the  thickness.  The 
curves  had  very  sharply  marked  maxima  at  thicknesses  of  about 
10  ~7  cm. 

In  1919,  two  independent  investigations  of  a  more  elaborate 
type  than  any  of  the  previous  investigations  on  thin  films  were 
published.  The  results  differ  in  several  respects.  Stuhlmann4 

1  P.  R.,  2,  199  (1913);  2,  327  (1913). 

2  Ibid.,  3,  195  (1914). 

«  P.  M.,  32,  421  (1916). 
«  P.  R.,  13,  109  (1919). 


120 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


produced  his  thin  films  by  a  novel  method.  They  were  obtained 
by  the  evaporation  in  vacuo  from  a  wire,  made  white-hot,  and 
placed  to  one  side  of  a  plate  of  quartz.  Thus  a  deposit  of  decreasing 
thickness  was  obtained  on  the  quartz  and  the  photo-electric  effect 
of  any  thickness  could  be  studied  by  directing  the  light  on  to  the 
proper  part  of  the  deposit.  The  metals  investigated  were  Ft  and 
Ag.  The  results  for  Ft  are  reproduced  in  the  diagram.  It  will 
be  seen  that  the  thickness  for  which  the  photo-electric  current  is  a 
maximum  decreases  with  the  frequency.  Since  all  the  curves  cut 
the  abscissa  at  the  origin  at  a  finite  angle,  the  tangent  here  may 

be  taken  to  measure  the 
photo-electric  effect  per  unit 
thickness  unaffected  by  any 
question  as  to  absorption  of 
light  or  of  electrons  in  the 
film.  Stuhlmann  concludes 
that  for  very  small  thick- 
nesses (7  X  10~7  cm.)  the 
electrons  pass  through  the 
film  colliding  according  to 
Rutherford's  hypothesis  of 
"single  scattering"  and  little 
or  no  energy  is  lost  by  the 
electron.  For  thicker  films, 
ordinary  "compound  scatter- 
ing" conies  into  play,  in  which 
the  electrons  lose  energy,  and 
for  still  thicker  films  true 
absorption  of  photo-electrons 


^8 
Thickness 

FIG.  12. 


takes  place.  By  comparison  of  the  results  for  Ft  and  for  Ag,  it 
was  concluded  that  the  stopping  power  of  a  metal  for  photo-elec- 
trons increases  as  the  energy  of  the  electron  increases  up  to  a  cer- 
tain limiting  value,  and  is  greater,  the  heavier  the  atom. 

Compton  and  Ross1  investigated  Ft  and  Au  films  produced 
by  sputtering.  It  may  be  useful  to  record  that  three  entirely 
different  methods  were  used  for  measuring  the  thicknesses  of  the 
films  and  gave  results  in  very  good  agreement.  The  form  of  the 
function  giving  the  relation  between  the  photo-electric  effect  and 
the  thickness  was  deduced  from  three  different  sets  of  assumptions. 

1  P.  R.,  13,  374  (1919). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


121 


These  were  as  follows:  (1)  That  the  number  of  the  photo-electrons 
which  retain  ability  to  escape  falls  off  exponentially  with  the  dis- 
tance moved  through  perpendicularly  to  the  surface;  (2)  that  the 
number  which  retain  ability  to  escape  falls  off  exponentially  with 
the  distance  moved  through  in  any  direction;  and  (3)  that  an 
electron  loses  energy  in  proportion  to  the  distance  moved  through 
the  metal.  The  thickness  of  the  film  giving  the  maximum  photo- 
electric effect  was  determined  theoretically  for  each  case  and  com- 
pared with  the  experimental  results.  The  results  were  found 
to  -fit  either  hypothesis  (1)  or  hypothesis  (2),  but  would  not  fit 
hypothesis  (3)  at  all.  (2)  is  probably  more  plausible  than  (1), 
90[ 


0     I      I 


5456     7_$.  9    10  II 
Thickness 

FIG.  13. 


as  the  electrons  are  as  likely  to  start  off  in  any  one  direction  as  in 
any  other.  One  set  of  curves  is  reproduced  here.  The  results 
differ  radically  from  those  of  Stuhlmann.  In  the  first  place,  they 
possess  two  maxima.  The  second  maximum  (the  one  for  the 
greater  thickness)  was  found  to  disappear  in  the  course  of  time, 
and  seems  to  be  associated  with  the  unstable  form  of  newly  sput- 
tered Pt.  The  single  maximum  in  Stuhlmann's  work  would  imply 
that  the  film  produced  by  evaporation  did  not  pass  through  the 
temporary  state  of  instability.  The  maxima  in  Compton  and 
Ross's  work  occur  at  about  4  X  10 ~7  cm.  for  X  2536  and  about 
2.5  X  10~7  cm.  for  X  2100  and  X  2225,  while  in  Stuhlmann's  work 
the  maxima  are  at  about  15  X  10~7  cm.  for  X  2536,  and  about 
12  X  10~7  cm.  for  X  2260.  Since  the  experiments  are  in  agree- 


122 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


ment  with  the  view  that  the  ability  to  escape  decreases  exponentially 
with  the  distance  travelled,  the  average  distance  through  which 
an  electron  can  travel  (a  sort  of  mean  free  path)  may  be  calculated. 
The  values  are  2.67  X  10 ~7  for  Pt,  and  5.0  X  10 ~7  for  Au,  which 
are  greater  than,  but  of  the  same  order  of  magnitude  as,  the  dis- 
tance between  the  atomic  centers  in  these  metals.  The  most 
important  result  of  this  investigation  perhaps  is  that  the  ability 
to  escape  is  the  same  for  a  fast  photo-electron  (i.  e.,  one  produced 
by  the  shortest  wave-length)  as  for  a  slow  photo-electron.  This 
can  only  be  accounted  for  on  the  view  that  a  photo-electron  loses 
its  energy  completely  at  a  single  collision,  and  not  gradually  during 
a  succession  of  collisions,  for  in  the  latter  case  it  is  obvious  that 
the  faster  electron  would  travel  further.  A  similar  conclusion 
may  be  drawn  from  Stuhlmann's  work,  although  in  his  experiments 
the  "mean  free  path"  of  an  electron  in  the  metal  appears  to  be 
greater  than  indicated  by  the  work  of  Compton  and  Ross. 


CHAPTER  VII 

PHOTO-ELECTRIC  EFFECTS  OF  NON-METALLIC   ELEMENTS 
AND  INORGANIC  COMPOUNDS 

Dima1  investigated  the  photo-electric  effect  of  numerous  in- 
organic compounds.  These  were  in  the  form  of  capsules  of  com- 
pressed powder.  The  light  used  was  that  from  a  quartz  mercury 
lamp,  unresolved.  As  the  substances  showed  fatigue  in  widely 
varying  amounts,  the  initial  values  of  the  photo-electric  effect 
were  taken  so  as  to  make  a  fair  comparison.  The  values  of  the 
initial  photo-electric  current  obtained  from  the  various  compounds 
under  similar  conditions  are  as  follows : 


HgI2                       10 

Hgl 

112 

CuO 

4800 

Cu,O 

14400 

HgCU                     2 

HgCl 

12 

CuCl. 

10 

CuCl 

50000 

HgO                      70 

Hg20 

280 

Pb02 

1700 

PbO 

3200 

Hg(C6Hf,C02)2      12 

Hg(C6H&C02) 

18 

CrO3 

1 

Cr20, 

50 

SnO2                      24 

SnO 

1220 

BiO3 

70 

Bi20, 

110 

SnS*                    186 

SnS 

1440 

Mn02 

48 

Mn5O4 

130 

Fe20s                   202 

FeO 

7200 

MnO 

500 

Fed,                       1 

FeCl2 

26 

It  is  evident  from  the  table  that  when  a  metal  can  combine 
with  another  element  in  two  ways,  that  compound  in  which  the 
metal  has  the  lower  valency  has  the  bigger  photo-electric  effect. 

1  C.  R.,  176,  1366  (1913);  177,  590  (1913). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


123 


This  view  was  verified  by  experiments  on  the  photo-electric  effects 
of  MnO,  Mn3O4,  Mn2O3,  MnO2,  which  were  400,  240,  176  and  37, 
respectively.  It  was  noticed  that  the  photo-electric  fatigue  was 
always  greater  in  a  compound  in  which  the  metal  has  a  low  valency 
(i.  e.,  in  the  one  with  the  bigger  photo-electric  effect)  than  in  the 
corresponding  compound  in  which  it  has  a  higher  valency.  Thus, 
the  photo-electric  effect  of  PbO  decreased  by  60  per  cent  in  twenty 
minutes,  while  that  of  PbO2  remained  constant  for  a  period  of  over 
three  hours.  It  is  probable  that  the  fatigue  is  associated  with  the 
conversion  of  the  surface  of  the  compound,  by  illumination,  into 
the  more  stable  compound  in  which  the  metal  has  a  higher  valency 
and  whose  photo-electric  effect  is  smaller.  In  a  few  cases,  e.  g., 
molybdenum  trioxide,  the  photo-electric  effect  increased  with 
continued  illumination.  Dima  suggested  that  the  light  effected  a 
chemical  reduction  in  such  cases.  The  following  results  were 
obtained  for  a  number  of  halides : 


Chloride 

Bromide 

Iodide 

K  

67 

320 

1200 

Pb  

31 

97 

3000 

Hg  (—  ous)  

15 

19 

1400 

Hg  (—  ic)  

5 

14 

230 

Ag  

200 

430 

750 

Cd  

60 

24 

18 

With  the  single  exception  of  the  compounds  of  Cd,  it  will  be  seen 
that  the  photo-electric  effect  increases  as  a  heavier  halogen  atom 
is  substituted  for  a  lighter. 

Naccari1  investigated  the  effect  of  light  on  the  transmission  of 
electricity  through  toluene  between  a  plate  and  a  gauze  1  mm. 
in  front  of  it.  The  fact  that  the  increased  conductivity  due  to 
illumination  was  independent  of  the  direction  of  the  field  seemed 
to  indicate  that  a  volume  ionization  was  produced  in  the  liquid 
rather  than  a  surface  effect  at  the  electrodes. 

La  Rosa  and  Cavallaro2  investigated  the  effect  of  illumination 
on  the  transmission  of  electricity  through  a  number  of  liquids. 
Surface  effects  and  volume  effects  were  generally  superposed  in 
varying  proportions.  Thus,  in  water,  alcohol  and  acetic  ether, 
the  surface  effect  predominated,  while  in  ethyl  ether  and  methylene 
bromide,  the  volume  effect  predominated. 

1  N.  Cimento,  4,  232  (1912). 

2  Ibid.,  6,39  (1913). 


124  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

Kelly1  employed  a  method  particularly  suited  to  the  study  of  the 
photo-electric  effect  of  insulators.  The  insulator  was  atomized 
in  liquid  form  (either  molten  or  in  a  solution),  and  then  passed 
into  an  apparatus  of  the  type  used  by  Millikan  in  his  determina- 
tion of  "e."  The  behavior  of  one  of  the  particles  so  produced 
was  studied  when  in  between  the  plates  of  the  condenser.  The 
particle,  when  initially  charged,  as  was  almost  always  the  case,. 
could  be  held  at  rest  by  an  electric  field  acting  in  opposition  to- 
gravity.  If,  now,  a  beam  of  ultraviolet  light  were  passed  into 
the  apparatus,  the  loss  of  photo-electrons  by  the  particle  would 
alter  its  charge  and  destroy  the  equilibrium.  Just  as  in  Millikan's. 
work  on  "e,"  the  charge  lost  could  be  measured.  It  was  found 
that  with  sufficiently  low  light  intensities,  the  emission  of  elec- 
tricity from  the  particles  consisted  in  the  emission  of  electrons 
one  by  one.  There  is  no  reason  to  suppose  that  this  does  not 
hold  with  greater  intensities  of  light,  though  it  is  difficult  to  dem- 
onstrate that  this  is  so  when  the  electrons  are  emitted  copiously. 
The  method  was  used  to  determine  the  photo-electric  thresholds 
for  insulators.  They  are  as  follows:  Sulphur <X  2400,  >  X  2200. 
Shellac  <  X  2200.  Oil  and  Paraffin  <  X  2150. 


CHAPTER  VIII 

PHOTO-ELECTRIC  EFFECTS  OF  DYES,   FLUORESCENT  AND 
PHOSPHORESCENT  SUBSTANCES 

The  only  recent  work  to  be  recorded  in  this  chapter  is  that  due 
to  Schmidt.2  He  investigated  the  effect  of  light  on  the  "phosphor" 
CaBiNa.  In  the  dark  this  is  a  dielectric,  under  the  influence  of 
light  it  becomes  conducting.  When  a  sufficiently  thin  layer  is 
used,  so  that  the  light  can  penetrate  throughout  the  material 
down  to  the  metal  which  acts  as  its  support,  a  current  can  be 
passed  through  it  when  illuminated.  This  is  known  as  the  actino- 
dielectric  effect.  The  curve  connecting  the  conductivity  with  the 
frequency  of  the  light  had  a  maximum  at  X  5800  and  a  minimum 
at  X4300  (thickness  used  .01  mm.).  If  thick  layers  are  used 
(e.  g.,  .5  mm.)  several  maxima  are  obtained.  These  are  determined 
more  by  the  absorption  characteristics  of  the  substances  than  by 
the  actinodielectric  properties,  hence  the  necessity  for  thin  films. 

1  P.  R.,  16,  260  (1920). 
*  A.  d.  P.,  44,  477  (1914). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  llV) 

Compton  and  Smyth1  have  shown  that  fluorescing  iodine  vapor 
is  more  easily  ionized  than  non-fluorescing  iodine  vapor.  Further 
mention  of  this  paper  will  be  found  in  the  chapter  on  ionizing 

potentials. 


CHAPTER  IX 

POSITIVE  RAYS  PRODUCED  BY  LIGHT 

No  further  work  appears  to  have  been  done  on  this  subject. 
It  seems  worth  while  to  examine  the  matter  more  thoroughly  and 
to  determine  the  nature  of  the  positive  carriers  should  their  existence 
be  verified. 


CHAPTER  X 

SOURCES  OF  LIGHT  USED  IN  PHOTO-ELECTRIC 
EXPERIMENTS 

Among  reliable  sources  of  ultraviolet  light  which  have  come 
into  regular  use  in  photo-electric  work  since  1913  may  be  mentioned 
the  Cooper  Hewitt  quartz  mercury  lamp.  For  constant  illumina- 
tion, such  sources  are  very  satisfactory.  The  shortest  wave-length 
available  is  X  1849,  owing  to  the  absorption  of  the  quartz.  Con- 
siderably shorter  wave-lengths  can  be  obtained  from  arcs  between 
metals  in  vacuo  and  from  discharges  in  gases.  Lyman2  has  in- 
vestigated the  extent  into  the  ultraviolet  of  the  spectrum  of  a 
discharge  through  helium  and  hydrogen  and  other  gases.  The 
hydrogen  spectrum  ends  at  about  X  905,  and  the  helium  spectrum 
at  about  X  510.  Similar  results  were  obtained  by  a  different 
method  by  Richardson  and  Bazzoni.  Lyman  also  investigated  the 
spectra  of  sparks  between  various  metals  in  his  vacuum  spectroscope. 
In  no  case  did  he  get  the  spectrum  to  extend  beyond  about  X  2000. 
McLennan  and  Lang3  investigated  the  extent  of  the  spectrum 
emitted  from  arcs  of  various  metals.  The  Hg,  Fe  and  Cu  spectra 
extended  to  X  1435,  X  1427,  and  X  1925,  respectively.  Carbon 
showed  a  much  shorter  line,  viz.,  X  584. 

Millikan  and  Sawyer4  found  that  sparks  between  metallic  elec- 

1  Science,  51,  571  (1920). 

2  "The  vSpectroscopy  of  the  Extreme  Ultra-Violet,"  Longmans;  also  A.  P.  J.,  43, 
89  (1916);  Science,  45,  187  (1917). 

3  P.  R.  S.,  95,  258  (1919). 

4  P.  R.,  12,  167  (1918);  A.  P.  J.,  52,  47  (1920). 


126  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

trodes  very  close  together  in  an  extremely  good  vacuum  gave 
spectra  extending  far  into  the  ultraviolet.  With  electrodes  of 
C,  Zn,  Fe,  Ag  and  Ni,  the  respective  spectra  extend  to  X  360.5, 
X  317.3,  X  271.6,  X  260.0,  X  202.0. 

If  one  wishes  to  carry  out  photo-electric  experiments  with  mono- 
chromatic light  of  wave-lengths  below  about  X  1850,  some  type  of 
vacuum  spectrometer  of  the  kind  used  in  the  investigations  just 
cited  must  be  used.  Sabine  investigated  the  velocities  of  photo- 
electrons  by  very  short  wave-lengths  isolated  in  a  vacuum  spec- 
trometer. Such  experiments  are  very  troublesome  to  carry  out 
on  account  of  the  necessity  of  having  the  source  of  light,  the  grating, 
and  the  apparatus  containing  the  illuminated  electrode,  all  in  the 
same  vacuum. 

The  monochromatic  illuminator  made  by  Hilger  is  useful  for 
isolating  any  portion  of  the  spectrum  from  the  visible  to  X  1850. 
The  light  which  passes  through  is  not  absolutely  monochromatic 
on  account  of  a  small  amount  of  unavoidable  scattering  of  light 
by  the  lenses  and  the  interior.  In  some  special  cases,  it  has  been 
found  advisable  to  use  light  niters  to  help  to  cut  out  all  the  light 
except  that  of  the  particular  wave-length  desired. 

For  many  purposes,  where  an  extremely  narrow  range  of  wave- 
lengths is  not  required,  light  filters  may  advantageously  take  the 
place  of  a  monochromatic  illuminator  in  the  visible  region.  A 
greater  intensity  of  light  is  usually  available  than  with  the  instru- 
ment. In  addition  to  the  list  of  light  filters  given  in  the  writer's 
"Photo-Electricity"  may  be  mentioned  the  numerous  light  niters  put 
out  by  the  Eastman  Company.  Being  made  of  gelatine  or  colored 
glass,  they  are  much  more  convenient  than  solutions.  One  series 
of  niters  is  particularly  adapted  for  use  with  the  mercury  lamp, 
the  range  of  transparency  not  including  more  than  one  or  two 
lines,  and  so  practically  giving  monochromatic  light.  (A  filter 
is  made  for  use  with  the  green  line  of  mercury  X  5467  which  trans- 
mits about  50  per  cent  of  this  and  is  quite  opaque  to  all  the  rest 
of  the  mercury  spectrum.)  It  is  unfortunate  that  a  similar  series 
of  light  filters  has  not  as  yet  been  produced  for  use  in  the  ultra- 
violet. The  Eastman  series  includes  one  ultraviolet  filter. 

Methyl  alcohol  has  a  sharply  defined  absorption  band  beginning 
at  X  2350  according  to  Hagenow.1  Kelly2  found  that  cobalt  chloride, 
dissolved  in  methyl  alcohol,  had  a  fairly  well-defined  transmission 

1P.  R.,  13,415  (1919). 
2  Ibid.,  16,  260  (1920). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  127 

region  in  the  ultraviolet  from  X  2650  to  X  4600  for  a  path  8  mm. 
long  through  a  2-normal  solution.  The  lower  limit  was  extended 
to  X  2400  for  a  .01  normal  solution. 

Information  as  to  new  substances  available  for  use  as  "windows" 
in  photo-electric  work  has  been  obtained  recently  by  Miss  Laird.1 
Silver  foil  .00002  cm.  thick  transmitted  fairly  well  down  to  X  1140, 
and  faintly  to  X  900.  Celluloid  films  (.02  mg./cm.2)  transmitted 
easily  to  X  900  and  faintly  to  X  600,  and  there  was  no  direct  indi- 
cation of  a  lower  limit. 

Miethe  and  Stenger2  give  a  list  of  the  transparency  regions  in 
the  ultraviolet  for  solutions  of  tartrazine,  filter  yellow,  Martin's 
yellow,  fluorescin,  eosin,  and  nitrosodimethylaniline.  The  short 
wave-length  limits  of  these  substances  range  from  about  X  3200 
to  about  X  2600.  Several  of  them  give  narrow  transmission  regions 
when  concentrated,  e.  g.,  tartrazine  transmits  a  band  from  X  3000 
to  X  3080. 

Lewis3  finds  that  benzol  transmits  down  to  X  1900. 


CHAPTER  XI 

IONIZING  AND  RADIATING  POTENTIALS 
EXPERIMENTAL  METHODS 

Most  of  the  phenomena  of  photo-electricity,  as  hitherto  con- 
sidered, deal  with  the  separation  of  electricity,  when  matter  in 
some  form  or  another  is  illuminated  by  light  of  a  suitable  kind. 
The  inverse  effect,  the  production  of  light  by  the  passage  of  elec- 
tricity through  matter,  is  a  vast  subject,  including  the  whole  field 
of  spectroscopy,  and  will  not  be  considered  here.  However,  there 
is  one  part  which  is  so  intimately  connected  with  photo-electricity 
(in  its  ordinary  connotation)  and  which  brings  out  the  quantum 
relations  similar  to  those  underlying  photo-electricity  so  clearly 
that  it  is  natural  to  include  it  here.  That  part  is  usually  designated 
as  the  subject  of  ionizing  and  radiating  potentials.  A  remarkable 
advance  has  been  made  in  our  knowledge  in  the  subject  during 
the  past  two  or  three  years. 

When  a  molecule  is  struck  by  a  moving  electron,  the  collision 
may,  or  may  not,  be  an  elastic  one.  By  an  elastic  collision  is 

1  P.  R.,  15,  .543  (1920). 

2  Zeits.  Wiss.  Phot.,  19,  57  (1919);  Sc.  Abs.,  1920,  981. 

3  P.  R.,  16,  367  (1920). 


128  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

meant  one  in  which  the  electron  rebounds  with  a  negligible  transfer 
of  energy.  (If  the  mass  of  the  molecule  were  infinite  compared 
with  that  of  the  electron,  there  would  be  no  transfer  of  energy.) 
If  there  is  a  transfer  of  energy  to  a  monatomic  molecule,  there 
may  be  complete  ionization  as  shown  by  the  production  of  positive 
and  negative  ions,  or  there  may  be  "partial  ionization,"  i.  e.,  a 
disturbance  of  the  atom,  which  is  not  detectable  as  ionization 
but  is  shown  by  the  production  of  radiation.  In  the  cases  of 
polyatomic  molecules,  collisions  are  more  or  less  inelastic,  the 
transferred  energy  presumably  being  used  up  in  increased  motion 
of  the  component  atoms  relative  to  each  other.1 

The  ionizing  potential  is  the  least  potential  through  which  an 
electron,  starting  from  rest,  must  fall,  to  acquire  sufficient  kinetic 
energy  to  enable  it  to  ionize  a  normal  molecule  on  impact.  Simi- 
larly a  radiating  potential  measures  the  least  kinetic  energy  which 
an  electron  must  have,  so  that,  on  impact  with  a  molecule,  it 
may  emit  a  monochromatic  radiation  characteristic  of  the  molecule. 
(It  is  generally  agreed  that  the  radiation  occurs  afterwards  as  the 
molecule  returns  to  its  normal  state.)  There  may  be  several 
radiating  and  ionizing  potentials  depending  on  the  type  of 
ionization  and  radiation  produced.  Energy  in  excess  of  that 
corresponding  to  the  critical  potential  is  a  necessary,  but  by 
no  means  a  sufficient,  condition  that  ionization  or  radiation  may 
occur.  There  are  two  principal  ways  of  measuring  the  ionizing 
v  v  and  radiating  potentials.  The  "total  and 

partial  current"  method  is  as  follows. 

Let  F  (fig.   14)  be  a  source  of  electrons 
(generally  an  incandescent   filament),    G   a 
gauze,  and  P  a  plate,  in  a  gas  at  a  suitable 
'     pressure,  generally  between  1  mm.  and  .01 
mm.     If  the  total  current  to  the  gauze  and 
plate  combined  be  measured  as   a  function 
of  the  accelerating  potential  VA,  the  curve 
FIG  14  '  ™^   ^e    a   smooth  curve    up    to   a  certain 

point,  where  its  slope  will  increase  abruptly 
(fig.  15).  This  indicates  that  the  electrons  from  F  have  acquired 
sufficient  energy  from  the  electric  field  to  ionize  some  of  the  molecules 
with  which  they  collide.  If  now  it  is  arranged  so  that  there  is  a  small 
field,  VR,  between  the  gauze  and  the  plate,  retarding  the  electrons 
passing  through  the  gauze,  the  current  to  the  plate  is  called  the 

1  Compton  and  Benade,  P.  R.,  8,  449  (1916). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


129 


partial  current.  Now  as  long  as  the  collisions  are  elastic,  the 
electrons  in  their  journey  from  F  to  G  will  acquire  kinetic  energy 
determined  by  their  position  in  the  field.  Hence,  the  current 
received  by  P  will,  in  general,  remain  constant  or  increase  regularly 
as  the  accelerating  potential  is  increased.  If,  however,  the  electrons 
are  accelerated  until  they  make  inelastic  collisions,  then  those  which 
get  through  the  gauze  will  have  a  smaller  velocity  than  before,  and  so 
the  field  V»  is  able  to  stop  a  greater  proportion  than  before  inelastic 
collisions  set  in.  The  dips  in  the  curves  then  tell  us  when  inelastic 
collisions  take  place.  When  the  gas  pressure  is  high  enough  to 
ensure  that  most  electrons  make  several  collisions  between  F  and 
G  (fig.  16),  then  a  dip  will  occur,  under  suitable  conditions,  at 


O      1 


4-567 
Accel.    Pot. 

FIG.  15. 


8    9 


J 


0      I 


13      + 
Accel.  Pot 

FIG.  16. 


every  multiple  of  the  value  of  the  critical  potential.  Collisions 
are,  of  course,  inelastic  when  ionization  takes  place,  for  there  is  a 
transfer  of  energy  to  effect  the  ionization.  If  an  inelastic  collision 
is  indicated  by  the  partial  current  method,  but  no  ionization  is 
indicated  by  the  total  current  method,  it  is  concluded  that  at 
this  point  radiation  sets  in.  This  is  inferred  from  the  good  agree- 
ment between  experiments  of  this  kind  and  those  showing  directly 
the  presence  of  radiation.  Hence,  when  it  is  convenient  to  use 
the  "total  and  partial"  current  method,  one  may  infer  radiation 
from  the  dips  in  the  partial  current  curves.  The  use  of  the  partial 
current  curves  is  due  to  Franck  and  Hertz.1  It  is,  of  course, 
possible  that  other  phenomena  (e.  g.,  dissociation)  besides  radiation 
may  be  accompanied  by  inelastic  collisions.  Should  there  be 
1  V.  d.  D.  P.  G.,  16,  10  (1914). 


130 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


two  radiating  potentials,  as  in  the  case  of  mercury  (4.9  and  6.7 
volts  corresponding  to  X  2536  and  X  1849),  it  may  well  be  that 
the  "total  and  partial"  method  and  the  more  direct  methods 
do  not  show  up  the  two  critical  points  equally  well. 

The  Lenard  method  is  to  accelerate  electrons  from  a  source,  F, 
to  a  gauze,  G,  after  which  they  pass  into  a  retarding  field,  VR,  which 
is  greater  than  VA,  so  that  no  electrons  get  across  to  P  (fig.  17). 
If  the  electrons  on  passing  through  G  have  acquired  sufficient 
energy  to  ionize  molecules  after  passing  through  the  gauze,  the 
positive  ions  so  produced  are  driven  into  P.  The  method  then 
consists  in  noticing  (usually  by  a  sensitive  electrometer)  the  point 
at  which  positive  ions  can  be  detected  as  VA  is  increased.  Inas- 
much as  this  method  marks  the  ionizing  potential  by  noting  the 


\G 


G      G' 


FIG.  17. 


FIG.  18. 


beginning  of  a  current  of  positive  ions,  while  the  total  current 
method  marks  it  by  an  increase  in  a  negative  current  already 
existing,  it  should,  in  general,  be  the  more  sensitive  method.  This 
method,  which  was  the  one  principally  used  in  earlier  investiga- 
tions, did  not  distinguish  between  the  ionizing  potential  and  the 
radiating  potential,  for  the  radiation  falling  on  P  would  produce 
a  photo-electric  effect  at  P  and  so  cause  it  to  emit  photo-electrons, 
leaving  it  charged  positively,  which,  of  course,  is  exactly  what 
happens  when  positive  Ions  are  driven  to  it.  Davis  and  Goucher1 
introduced  a  method  whereby  the  ionizing  potential  and  radiating 
potential  could  be  separated.  A  second  gauze,  G',  was  introduced 
as  shown  (fig.  18)  so  that  a  small  field,  Vi  sufficient,  if  properly 
directed,  to  prevent  the  emission  of  photo-electrons,  could  be 
thrown  on,  to  the  right,  or  to  the  left,  as  one  chooses.  The  essential 
1  P.  R.,  10, 101  (1917). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


131 


Mei~cun 


>*; 


idea  of  the  method  is  this.  P  can  only  emit  photo-electrons, 
and,  therefore,  acquire  a  positive  charge,  when  the  field  is  such 
as  to  accelerate  them  away  from  it.  When  the  field  is  reversed 
these  photo-electrons  cannot  escape.  On  the  other  hand,  the 
photo-electrons  produced  by  the  radiation  reaching  the  right- 
hand  side  of  the  gauze  G'  (by  reflection  or  otherwise)  are  now 
enabled  to  pass  over  to  P  and  so  to  give  it  a  negative  charge.  Con- 
sequently, the  charge  acquired  by  P  changes  sign  as  the  direction 
of  Vi  is  changed  so  far  as  radiation  is  concerned.  Provided  that 
the  pressure  is  not  too  large,  and 
that  the  difference  of  potential  Vi 
between  G'  and  P  is  considerably 
less  than  VR,  the  positive  ions  pro- 
duced between  G  and  G'  will  reach 
P  whether  the  field  between  G'  and 
P  helps  or  hinders  them,  for  their 
velocity  on  reaching  G'  is  sufficient 
to  overcome  the  effect  of  the  field. 
Hence,  the  charge  acquired  by  P 
does  not  change  sign  as  the  direction 
of  Vi  is  changed,  so  far  as  the  posi-  +• 
tive  ions  are  concerned.  A  typical 
curve  is  shown  in  fig.  19.  Theradiat-  c 
ing  potential  corresponds  to  the  point  £- 
where  the  (I  +  R)  and  (I  -  -  R)  g 
curves  diverge,  while  the  ionizing 
potential  corresponds  to  the  points  ' 
where  there  is  an  abrupt  upward 
deflection  in  both  curves. 

A  second  modification  of  the 
Lenard  method,  to  distinguish 
radiating  potentials  and  ionizing  potentials  is  due  to  Compton.1 
The  collecting  electrode  is  a  cylinder  closed  at  one  end  by  a  gauze, 
G',  and  at  the  other  by  a  plate,  P  (fig.  20).  This  cylinder  can  be 
rotated  about  an  axis  as  shown,  so  that  G'  or  P  faces  the  gauze  G. 
The  essential  idea  of  this  method  lies  in  the  fact  that  the  positive 
ions  will  contribute  the  same  charge  to  the  cylinder,  whether  the 
gauze  end  or  the  plate  end  faces  the  gauze  G.  Radiation  pro- 
duces a  larger  effect  when  P  faces  G  than  when  G'  faces  G,  for 

1  P.  M.,  40,  553  (1920). 


TB1 


FIG.  19. 


132 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


/.xij   of 
Rotation 


\ 
I 

I 

'G' 


FIG.  20. 


in  the  latter  case  much  of  the  radiation  passes  through  the  gauze 
and  the  photo-electrons  produced  inside  do  not  escape  and  so 
contribute  nothing  to  the  charge  acquired  by  the  cylinder.  Typical 

curves  for  He  are  shown  in  fig.  21. 
A  represents  the  current  to  the 
cylinder  (P  facing  G)  as  a  function 
of  the  accelerating  potential.  B 
represents  the  current  when  G' faces 
G.  R  is  the  ratio  of  the  currents 
when  P  faces  G  to  the  case  when  G ' 
faces  G.  From  20  to  25  volts  the 
ratio  is  constant,  while  from  25  volts 
onwards  it  falls  rapidly  correspond- 
ing to  the  increasing  effect  of  ioniza- 
tion  after  the  ionizing  potential  is  passed.  x  It  should  be  mentioned 
that  the  constant  value  for  R  between  20  and  25  volts  does  not 
imply  the  absence  of  ionization  (it  implies  a  constant  ratio  between 
the  amounts  of  ionization  and  radiation),  indeed  the  method  was 
devised  for  investigation  of  the  ionization  produced  as  a  secondary 
effect  of  radiation  under  special  conditions.  This  will  be  taken 
up  again  later. 

To  determine  the  exact  maximum  energy  of  the  electrons  used 
in  any  determination  of  a  critical  potential,  it  is  usual  to  take  a 
"velocity  distribution"  curve  at  the  same  time.  This  is  done 
by  accelerating  the  electrons 
from  F  to  G  (figs.  14,  17, 
18,  20)  and  measuring  the 
number  reaching  P  as  a  func- 
tion of  a  retarding  field  be- 
tween G  and  P.  The  differ- 
ence between  the  applied  ac- 
celerating potential  and  the 
retarding  potential  required 
to  stop  all  the  electrons  gives 
the  necessary  correction  to 
the  applied  potential.  The 
more  homogeneous  in  veloc- 
ity the  electron  stream,  the  sharper  are  the  discontinuities  in  the 
curves  showing  the  critical  potentials.  To  secure  homogeneous 
electron  streams,  some  investigators  have  avoided  the  variation 


600 
500 

I 

R 

f-60 
1  40 
I  20 
I.OO 

/ 

// 

A 

i 

I 

i 

100 

// 

^ 

ft 

^ 

^ 

—  -. 

26    28     JO-  Jt 

Vo/rs 

FIG.  21. 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


133 


in  speed  arising  from  the  fall  of  potential  along  the  filament  by 
means  of  a  rapidly  rotating  commutator  which  cuts  off  the  heating 
current  when  the  accelerating  potential  is  applied  and  vice  versa. 
Thus  the  filament  is  an  equipotential  surface,  while  electrons  are 
being  accelerated  away  by  the  field.  Other  investigators  (e.  g., 
Davis  and  Goucher)  have  used  an  equipotential  platinum  tube 
(coated  with  lime)  heated  internally  by  a  resistance  coil. 

An  idea  of  the  energy  distribution  among  the  electrons  as  they 
are  emitted  from  an  incandescent  filament  may  be  obtained  from 
the  following  table  due  to  Langmuir. 


Filament  temperature 

2400°  K 

1200°  K 

90  per  cent  have  energy  exceeding 

0.022  volt 

0.011  volt 

75  per  cent  have  energy  exceeding 

0.059  volt 

0.030  volt 

50  per  cent  have  energy  exceeding 

0.143  volt 

0.071  volt 

25  per  cent  have  energy  exceeding 

0.29    volt 

0.145  volt 

10  per  cent  have  energy  exceeding 

0.48    volt 

0.24    volt 

1  per  cent  has  energy  exceeding 

0.95    volt 

0.47    volt 

0  .  1  per  cent  has  energy  exceeding 

1.42    volts 

0.71    volt 

0.0001  per  cent  has  energy  exceeding 

2.85    volts 

1.45    volts 

Thus,  a  more  homogeneous  stream  of  electrons  is  obtainable 
with  a  low  temperature  source. 

If  we  infer  the  energy  of  the  fastest  electrons  in  an  electron 
stream  from  the  point  where  the  velocity  distribution  curve  cuts 
the  potential  axes,  and  assume  that  this  energy  is  that  of  the  elec- 
trons which  are  responsible  for  the  first  observable  ionization  (or 
radiation)  the  critical  potentials  so  deduced  will  not  be  exactly 
correct.  The  reason  for  this  is  that  the  proportion  of  collisions 
resulting  in  ionization  (or  radiation)  is  very  small  just  above  a 
critical  potential,  and  even  if  it  amounted  to  its  maximum  possible 
value,  the  geometrical  arrangement  of  the  apparatus,  and  the 
fact  that  at  low  pressures  the  electrons  do  not  all  collide  with 
molecules  in  the  space  where  they  have  kinetic  energy  above  the 
critical  value,  would  prevent  its  full  value  being  registered.  Hence 
it  is  that  the  fastest  electrons  which  are  present  in  only  just  suffi- 
cient numbers  to  give  an  indication  on  the  velocity  distribution 
curve,  cannot  give  a  measurable  ionization  or  radiation  if  the 
same  indicating  instrument  be  used  for  both  measurements.  This 
was  first  pointed  out  by  Smyth1  who  worked  out  a  meChod  of 

1  P.  R.,  14,  409  (1919). 


134  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

correcting  for  it.  For  details,  reference  must  be  made  to  the 
paper.  In  several  subsequent  investigations  efforts  have  been 
made  to  overcome  the  error  pointed  out  by  Smyth,  but  they  do 
not  in  all  cases  appear  to  be  adequate. 

An  exceedingly  simple  and  accurate  method  of  correcting  for 
initial  velocities  is  due  to  Franck  and  his  collaborators.  In  the 
partial  current  curve  showing  inelastic  collisions  (fig.  16)  the 
distance  between  two  corresponding  dips  gives  the  exact  value 
of  the  critical  potential,  and  by  comparing  this  with  the  distance 
between  the  first  dip  and  the  origin,  the  necessary  correction  to 
the  applied  potential  is  obtained. 

COLLECTED  RESULTS 

The  experimental  results  are  given  on  the  following  pages.  In 
cases  where  the  same  gas  has  been  investigated  by  several  observers 
independently,  the  results  are  given  under  each  gas  to  facilitate 
comparison.  In  other  cases,  where  a  number  of  gases  have  been 
examined  under  identical  conditions,  it  was  thought  advisable  to 
collect  them  together,  e.  g.,  the  results  of  Foote,  Mohler,  and 
collaborators,  and  of  Hughes  and  Dixon. 

It  should  be  added  that  in  many  cases,  investigators  have  used 
the  Lenard  method  and  have  not  distinguished  between  the  ionizing 
potentials  and  the  radiating  potentials.  Though  in  many  of  the 
earlier  researches,  the  critical  potentials  observed  have  been  called 
ionizing  potentials,  in  view  of  our  recent  knowledge  it  is  possible 
that  some  of  them  may  be  radiating  potentials  and  they  are,  there- 
fore, marked  with  an  *,  unless  the  investigator  definitely  decided 
against  radiation  as  an  adequate  explanation. 

Hydrogen  R.  P.  I.  P. 

Franck  and  Hertz  (V.  d.  D.  P.  G.,  15,  34 

(1913))  11* 

Goucher  (P.  R..  7,  561  (1916))  10.25* 

Hughes  and  Dixon  (P.  R.,  10,  495  (1917))    10.2* 

Bishop  (P.  R.,  9,  567  (1917))                          11*  15.8 

Davis  and  Goucher  (P.  R.,  10,  101  (1917))  11  11  («.  e.,  both  at  11  volts) 

15. 8  2nd  type 

13.6  ..     2nd  type 

Horton  and  Davies  (P.  P..  S.,  97,  23  (1920))  10. 5  14 .4  (atom) 

13.9  16.9  (molecule) 

Mohler  and  Foote  (/.  0.  S.  A,  4, 49  (1920);  10.4  13.3  (atom) 

Bur.  Stan.,  1920,  670)  12.22  16.5  (molecule) 

Stead  and  Gossling](P.  M.,  40,  413  (1920))       . .  15 
Franck,  Knipping  and  Kriiger  (V.  d.  D.  P. 

G.,  21,  728  (1919))                                         . .  11.5  (molecule) 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


135 


R.P. 

13.6 


Found  (P.  R.,  16,  41  (1920)) 

Compton   and   Olmstead    (P.    R.,  17,  45 

(1921))  10.8 

13.4 

Helium 

Franck  and  Hertz  (V.  d.  D.  P.  G.,  15, 

34  (1913))  21* 

Bazzorii  (P.  M.,  32,  566  (1916)) 

Rentschler  (P.  R.,  14,  503  (1919))  none 

Franck  and  Knipping  (P.  Z.,  20, 481  (1919) ; 

V.  d.  D.  P.  G.,  20,  181  (1919))  20.5 


Franck   and   Knipping    (Z.  /.    P.,    i, 
320  (1920)) 

Horton   and    Davies    (P.    R.    S.,   95, 

408  (1919);  P.M.,  39,  592  (1920)) 


(20.45) 


21.25 


20.5 
41 


Found  (P.  R.,  16,  41  (1920)) 

Stead  and  Gossling  (P.  M.,  14,  413  ( 1920)) 

Compton  (P.  M.,  40,  553  (1920))  20.2 

Argon 

Franck  and  Hertz  (V.  d.  D.  P.  G.,  15, 

34  (1913))  12* 

Rentschler  (P.  R.,  14,  503  (1919))  12 

Horton  and  Davies  (P.  R.  S.,  97,  1  (1920))     11.5 
Found  (P.  R.,  16,  41  (1920)) 
Stead  and  Gossling  (P.  M.,  40, 413  (1920)) 

Neon 

Franck  and  Hertz  (V.  d.  D.  P.  G.,  15, 34 

(1913))  16* 

Rentschler  (P.  R.,  14,  503  (1919))  none 

Horton   and   Davies    (P.   R.  S.,  98,  121 

(1920))  (11.8 

(17.8 

Nitrogen 

Franck  and  Hertz  (V.  d.  D.  P.  G.,  15, 

34  (1913))  7.5* 

Goucher  (P.  R.,  7,  561  (1916))  7.4* 

Hughes  and  Dixon  (P.  R.,  10,  495  (1917))  7.7* 


/.  P. 
.  .     (dissociation       and 

radiation) 
17.1  (dissociation       and 

ionization) 
30.45  (dissociation    and 

double  ionization) 
15.1 

10.8 
15.9 


20 

27 

25. 4  (normal atom) 

79.5  (double  ionization) 

(not   present  in   the   pur- 
est He) 
25.3 

25.7  (normal  atom) 

55      (charged  atom) 

80      (double   ionization) 

20.5 

20.8 

25.5 


17 

15.1 
15.6 
12.5 


21 

16.7) 
20.0^ 
22.8) 


136 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


I.  P. 


Bishop  (P.  R.,  9,  567  (1917)) 

Davis  and  Goucher  (P.  R.,  13,  1  (1919)) 

Karrer  (P.  R.,  13,  297  (1919)) 
Smyth  (P.  R.,  14,  409  (1919)) 


Sodium 

Hebb  (P.  R.,  12,  482  (1918)) 

Wood  and  Okano  (P.  M.,  34,  177  (1917)) 

Tate  and  Foote  (P.  M.,  36,  64  (1918)) 

Iodine 

Found  (P.  R.,  16,  41  (1920)) 

Mohler  and  Foote  (P.  R.,  15,  321  (1920)) 


18 


Found  (P.  R.,  16,  41  (1920)) 

Mohler  and  Foote  ( J.  O.S.A.,4, 49  (1920)) 

Stead  and  Gossling  (P.  M.,  40, 413  (1920)) 

Nitrous  Oxide 

Bishop  (P.  R.,  9,  567  (1917)) 

Oxygen 

Franck  and  Hertz  (V.  d.  D.  P.  G.,  13,  34 

(1913)) 

Hughes  and  Dixon  (P.  R.,  10, 495  (1917)) 
Bishop  (P.  R.,  10,  244  (1917)) 
Mohler  and  Foote  ( J.  O.S.A.,4,  49  (1920)) 

Mercury 

Goucher  (P.  R.,  8,  561  (1916)) 
Bishop  (P.  R.,  10,  244  (1917)) 
Hughes  and  Dixon  (P.  R.,  10,  495  (1917)) 
Tate  (P.  R.,  10,  81  (1917)) 

Davis  and  Goucher  (P.R.,  10, 101  (1917)) 

Hebb  (P.  R.,  u,  170  (1918)) 
Hebb  (P.  R.,  15,  130  (1920)) 
Found  (P.  R.,  15,  132  (1920)) 
Kingdom  (in  print) 

Stead  and  Gossling  (P.  R.,  40, 413  (1920)) 
Franck    and    Einsporn    (Z.  /.    P.,    2,    18 
(1920)) 


R.  P. 
7.5* 
7.5 
9.0 

No.  I.  P.  <  10  volts 

8.29  (strong)          18 

7.3    (doubtful)      ... 

6.29  (strong,  low   .  . 

pressures) 

15.8 
8.18  16.9 

17.2- 


7.5< 


9.0* 

9.2*  •     .. 

9.0* 

7.91  15.5 


4.9 
4.9 
6.7 
4.9 


(4.68 

(4.9 

(5.32 

(5.76 

(6.04 

(6.30 

(6.73) 


1.0 
0.5 
2.12 


2.34 


7.12) 
7.46) 
7.73) 
8.35) 
8.64) 
8.86) 


10.27 
10.2 
10.3 
10.4 

4.9 
3.2 

10.1 
4.9 

10.8 

10.38 


2.5 


.13 


8.5 
10.1 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


137 


Compton    and    Smyth    (Science,    51,    571 
(1920))     (P.  R.  16,  .501  (1920)) 


Carbon  Monoxide 

Hughes  and  Dixon  (P.R.,  io,495  (1917))         7.2* 

Found  (P.  R.,  16,  41  (1920)) 

Stead  and  Gossling  (P.  M.,  40,  413  (1920))    .  . 

Hydrochloric  Acid  Gas 

Hughes  and  Dixon  (P.  R.,  10, 495  (1917))        9 . 5' 
Foote  and  Mohler  (J.  A.  C.  S.,  49,  1821 
(1920)) 


8.0  (atom) 
9.4  (molecule) 
6.8  (fluorescing      mole- 
cule) 


15.0 
15.0 


13.7 


The  following  is  a  table  of  experimental  results  obtained  for  a 
large  number  of  metallic  vapors  (and  some  non-metallic  vapors  also) 
by  Foote,  Mohler,  Tate,  Stimson,  Meggers,  and  Rognley.  They 
were  obtained  chiefly  by  the  total  and  partial  current  method,  and 
in  view  of  the  fact  that  the  experiments  were  carried  out  on  a 
systematic  and  connected  plan,  it  was  thought  desirable  to  collect 
them  together. 

Na         Tate  and  Foote  (P.  M.,  36,  64  (1918)) 
K  Tate  and  Foote  (P.  M.,  36,  64  (1918)) 

Cd          Tate  and  Foote  (P.  M.,  36,  64  (1918)) 

Cd  Mohler,  Foote  and  Meggers  (Bur.  Stan.,  1920,  734) 

Zn  Tate  and  Foote  (P.  M.,  36,  64  (1918)) 

Zn  Mohler,  Foote  and  Meggers  (Bur.  Stan.,  1920,  734) 

Mg  Foote  and  Mohler  (P.  M.,  37,  33  (1919)) 

Mg  Mohler,  Foote  and  Meggers  (Bur.  Stan.,  1920, 734) 

Hg  Tate  (P.  R.,  10,  81  (1917)) 

Hg  Mohler,  Foote  and  Meggers  (Bur.  Stan.,  1920,  734) 

Ca          Mohler,  Foote  and  Stimson  (Bur.  Stan.,  1920,  368) 

Tl  Foote  and  Mohler  (P.  M.,  37,  33  (1919)) 

Pb          Mohler,  Foote  and  Stimson  (Bur.  Stan.,  1920,  368) 

Rb         Foote,  Rognley  and  Mohler  (P.  R.,  13,  61  (1919)) 

Cs          Foote,  Rognley  and  Mohler  (P.  R.,  13,  61  (1919)) 

As          Foote,  Rognley  and  Mohler  (P.  R.,  13,  61  (1919)) 

N2          Mohler  and   Foote  (/.  O.  S.  A.,  4,  49  (1920);    Bur. 

Stan.,  1920,  670) 
Oa  Mohler  and  Foote  (J.  O.  S.  A.,  4,  49  (1920);    Bur. 

Stan.,  1920,  670) 
H2          Mohler  and  Foote  (/.  0.  S.  A.,  4,  49  (1920);    Bur. 

Stan.,  1920,  670) 


R.  P. 

1.  P. 

2.12 

5.13 

1.55 

4.1 

3.88 

8.92 

f  3.95 

\5.35 

9.0 

4.1 

9.5 

J4.18 

9.3 

\  5.65 

2.65 

7.75 

f  2.65 

8.0 

\  4.42 

4.9 

10.3 

4.76 

10.2 

6.45 

1.90 

6.01 

2.85 

1.07 

7.3 

1.26 

7.93 

1.6 

4.1 

1.48 

3.9 

4.7 

11.5. 

8.18 

7.91 
10.4 
12.22 


16.9 

15.5 
13.3 
16.5 


138 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


P  Mohler  and  Foote  (P.  R.,  15,  321  (1920);  Bur.  Stan., 

1920,  670)  5.80  13.3 

I  Mohler  and  Foote  (P.  R.,  15,  321  (1920);  Bur.  Stan., 

1920,670)  2.34  10.1 

S  Mohler  and  Foote  (P.  R.,  15,  321  (1920);  Bur.  Stan., 

1920,  670)  4.78  12.2 

The  following  are  values  obtained  by  Hughes  and  Dixon1  for 
the  first  critical  potentials  for  a  number  of  gases.  In  view  of  our 
recent  knowledge,  it  is  probable  that  in  most  cases  they  refer  to 
radiating  potentials  and  not  to  ionizing  potentials. 


H2 

10.2  volts 

CO 

7.2  volts 

02 

9.2  volts 

C02 

10.0  volts 

Ni 

7.7  volts 

NO 

9.3  volts 

Cl 

8.2  volts 

CH< 

9  .  5  volts 

Hg 

10.4  volts 

C2H6 

10.0  volts 

HC1 

9  .  5  volts 

C2H4 

9.9  volts 

Br 

10.0  volts 

C2H2 

9.9  volts 

Richardson  and  Bazzoni2  carried  out  an  interesting  investiga- 
tion on  the  extreme  ultraviolet  radiation  emitted  by  He,  H2  and 
Hg,  when  electrons  of  velocities  up  to  800  volts  were  driven  through 
them.  The  radiation  produced  was  allowed  to  fall  on  a  metal 
surface  and  the  velocities  of  the  photo-electrons  emitted  were 
measured  by  a  magnetic  method.  The  fastest  photo-electrons 
in  each  case  gave  the  shortest  wave-length  in  the  radiation.  It  was 
found  that  the  shortest  wave-length  in  the  radiation  was  determined 
by  the  nature  of  the  gas  and  was  quite  independent  of  the  energy 
of  the  electrons  up  to  800  volts  (provided,  of  course,  that  the  mini- 
mum energy  corresponding  to  the  radiation  was  present).  These 
results  are  not  directly  comparable  with  those  on  the  radiating 
potential,  for  the  latter  indicate  the  first  line  in  the  series,  while 
these  experiments  indicate  the  shortest  line  of  appreciable  intensity 
in  the  series.  The  results  are  as  follows : 


Shortest  wave-length 

Corresponding  voltage 

H2 
He 
Hg 

>   X830     <  X950 
>   X420     <  X470 
>X1000     <X1200 

<14.8      >13.0 

<29.4     >25.7 
<12.4     >10.2 

It  will  be  seen  that  a  large  amount  of  evidence  as  to  the  ionizing 
and   radiating   potentials   has   been    amassed.     It    seems    certain 

'  P.  R.,  10,  495  (1917). 
*  P.  M.,  34,  285  (1917). 


REPORT  ON  PHOTO-ELECTRICITY;  A.  LL.  HUGHES  139 

that  each  gas  or  vapor  has  at  least  one  radiating  potential  and  one 
ionizing  potential,  both  clearly  marked.  Cases  in  which  ionization 
occurs  at  the  radiating  potentials  are  ignored  as  they  will  be  con- 
sidered later  as  examples  of  "cumulative  effects."  It  remains 
to  be  seen  how  these  results  can  be  accounted  for  theoretically. 
The  most  reliable  values  for  the  critical  potentials  can  be  associated 
with  certain  important  features  in  the  spectra  of  the  gases  in  the 
majority  of  cases.  For  hydrogen  and  helium,  Bohr's  theory  is 
found  to  link  up  most  of  the  results  satisfactorily,  while  in  the 
case  of  the  metals  for  which  series  are  known  in  some  detail,  con- 
sistent relations  can  be  found. 

BOHR'S  THEORY  FOR  HYDROGEN  AND  HELIUM 

Hydrogen.  According  to  Bohr,  the  negative  energy  of  the  hy- 
drogen atom  pictured  as  a  simple  system  of  one  positive  nucleus 
with  one  electron  rotating  around  it  in  a  circular  orbit,  is 


where  W0  is  the  negative  energy  of  the  innermost  possible  orbit 
(for  which  r  =  1)  and  r  is  an  integer.  (Only  those  orbits  for  which 
T  is  an  integer  are  possible,  on  Bohr's  theory.)  The  innermost  possi- 
ble orbit  (r  =  1)  corresponds  to  the  normal  hydrogen  atom.  When- 
ever the  atom  undergoes  a  change  in  configuration,  such  that  the 
atom  passes  from  orbit  r2  to  rl  (T^T^  an  amount  of  energy  is 
liberated  amounting  to 


An  essential  feature  of  Bohr's  theory  is  that  this  liberation  of 
energy  determines  the  frequency  of  the  monochromatic  light 
emitted  during  the  change,  through  the  quantum  relation 


or 


where  K  is  known  as  Rydberg's  number  which  equals  3.290  X  1015. 
To  each  pair  of  integral  values  for  TX  and  r2  corresponds  a  line, 


140  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

and  these  lines  fall  into  series,  each  series  having  different  values 
for  T!  and  each  member  in  a  series  having  a  different  value  for 
T2.  Spectroscopic  identification  of  the  various  series  has  led  to  a 
determination  of  K',  in  the  corresponding  "wave  number"  equation 


as  109678.3  correct  to  about  one  part  in  a  million.1  Our  K  (Ryd- 
berg's  constant)  is  related  to  K'  through  the  velocity  of  light 
(K  =  cK')  and  is  therefore  known  to  the  same  degree  of  accuracy 
as  the  velocity  of  light  is  known.  Its  value  is  3.290  X  10 15  as 
far  as  the  first  four  significant  figures. 

We  shall  now  proceed  to  calculate  the  potential  corresponding 
to  the  limit  (X0)  of  the  shortest  wave-length  series  in  hydrogen 
(T!  =  1 ,  T2  =  oo ).  This  frequency  VQ  is  important,  in  that 
through  the  quantum  relation  we  have  at  once  the  energy  re- 
quired to  remove  the  electron  from  the  innermost  orbit  (i.  e., 
from  a  normal  hydrogen  atom)  to  infinity,  in  other  words,  the 
energy  necessary  to  ionize  the  hydrogen  atom. 

V0e  =  hvQ  =  W0  =  Kh 
Hence 

V0  =  K - 

e 

-  3.290  X  10»  X  °-547  X  10"27  X  2"'86  volts 
4.774  X  10-10 

=  13.524  volts. 

The  corresponding  X0  is  911.74  Angstrom  units  (which  may  also 
be  identified  directly  as  the  reciprocal  of  Curtis's  value  for  the 
hydrogen  spectrum  constant). 

According  to  Bohr,  the  negative  energies  of  the  hydrogen  atom, 
the  hydrogen  molecule  (i.  e.,  two  separated  positive  nuclei  with 
two  electrons  rotating  symmetrically  around  the  line  joining  them 
as  axis)  and  the  charged  hydrogen  molecule  (i.  e.,  the  hydrogen 
molecule  just  referred  to  with  one  electron  removed)  are  as  follows : 

Neutral  hydrogen  atom W0 

Neutral  hydrogen  molecule 2 . 20  W0 

Charged  hydrogen  molecule 0.88  W0 

The  following  energies  are  required  to  effect  the  changes  indicated : 

1  Curtis,  P.  R.  S.,  96,  147  (1919). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  141 

Corresponding 
volts 

Neutral  atom  >  Charged  atom.. ..  W0  —  0  W0     (13.52) 

Neutral  molecule  — >•  charged  mole- 
cule      2.20  W0  —  0.88  W0  =  1.32  W0     (17.85) 

(Bohr  considers  this  less  likely  to  represent 
ionization  of  the  molecule  than  the  following) 

Neutral  molecule  — >  Neutral  atom 

and  charged  atom 2.20  W0  —  W0          =    1.20W0     (16.22) 

Neutral  molecule   >   Two  neutral 

atoms  (dissociation) 2.20  W0  —  2  W0      =  0.20  W0       (2.70) 

Neutral  atom  — >  Neutral  atom W0  J:  =  0.75  W0     (10.14) 

(Electron     shifts     orbit     1     >•    orbit     2 

giving  first  radiating  potential) 

Of  the  experimental  results  which  distinguish  between  radiating 
and  ionizing  potentials,  those  by  Mohler  and  Foote,  and  by  Horton 
and  Davies,  are  put  forward  as  being  in  fair  accord  with  the  pre- 
dictions of  Bohr's  theory.  It  is  by  no  means  clear,  however,  why 
experimental  investigations  on  ordinary  diatomic  hydrogen  should 
give  the  critical  potentials  predicted  for  atomic  hydrogen.  Al- 
though it  is  known  that  an  incandescent  filament  dissociates 
molecular  hydrogen,  and  that  atomic  hydrogen  is  present  with 
intense  electron  currents,  the  amount  of  atomic  hydrogen  in  most 
experiments  will  be,  at  the  most,  but  a  small  fraction  of  the  un- 
dissociated  hydrogen.  Hence  it  is  not  clear  why  critical  values 
associated  with  simple  collisions  between  atomic  hydrogen  and 
electrons  should  be  found. 

An  exceedingly  suggestive  point  of  view  has  been  put  forward 
by  Franck,  Knipping  and  Kriiger  in  support  of  their  experimental 
results.  According  to  them,  the  first  critical  potential  at  11.5 
volts  is  an  ionizing  potential  and  not  a  radiating  potential,  and 
it  is  maintained  that  this  corresponds  to  the  detachment  of  an 
electron  from  a  hydrogen  molecule.  That  a  positively  charged 
hydrogen  molecule  can  exist  is  shown  clearly  by  Sir  J.  J.  Thomson's 
work  on  positive  rays1  from  which  he  deduced  the  ionizing  potential 
of  the  hydrogen  molecule  to  be  11  volts,  a  valuable  result  in  that 
the  method  is  entirely  different  from  the  methods  here  considered. 
(From  energy  considerations,  Bohr  considers  that  the  formation 
of  the  positively  charged  hydrogen  molecule  is  less  likely  to  happen 
than  the  breaking  up  of  the  molecule  into  a  neutral  atom  and  a 
charged  atom.  This  gives  16.22  volts  instead  of  17.85  volts  (see 
table  above).  This  means  that,  on  Bohr's  theory,  no  positively 
charged  hydrogen  molecule  could  exist.)  Franck,  Knipping  and 

1  "Rays  of  Positive  Electricity,"  p.  36,  Longmans. 


142  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

Kriiger  explain  the  radiating  potential  which  they  obtained  at  13.6 
volts  as  the  simultaneous  dissociation  of  the  molecule  into  atoms  and 
the  displacement  of  the  electron  in  one  of  them  to  the  second  Bohr 
orbit,  so  that  it  is  in  a  position  to  give  out  radiation  of  frequency 
K  [l/l2  —  1/22]  corresponding  to  10.14  volts.  They  consider  3.5 
volts  to  represent  the  best  experimental  value  of  the  work  of  dis- 
sociating a  hydrogen  molecule.  (Langmuir's  value  is  3.6  volts.) 
Thus  the  radiating  potential  at  13.6  volts  is  taken  to  correspond  to 
dissociation  plus  radiation  from  an  atom  (theoretically  =  3.5  -f- 
10.14  =  13.6  volts).  Similarly  the  ionizing  potential  at  17.1 
volts  is  accounted  for  by  dissociation  plus  ionization  of  an  atom 
(theoretically  =  3.5  +  13.52  =  17.0  volts).  Again  the  ionizing 
potential  at  30.4  volts  is  accounted  for  by  dissociation  plus  ioniza- 
tion of  both  atoms  (theoretically  =  3. 5  +  2  X  13. 52  =  30. 5  volts). 
(On  the  same  lines  we  might  expect  a  radiating  potential  at  3.5  + 
2  X  10.14  =  23.8  volts,  but  none  is  recorded.)  In  support  of 
these  results,  Compton  and  Olmstead1  find  ionizing  potentials 
at  10.8  volts  and  at  15.9  volts,  and  a  radiating  potential  at  13.4 
volts.  They  find,  however,  radiation  at  10.8  volts,  contrary  to 
Franck,  Knipping  and  Kriiger.  The  proportion  of  radiation  to 
ionization  between  10.8  and  15.9  volts  depends  largely  on  conditions 
such  as  gas  pressure  and  electron  current.  It  may  be  that  cumu- 
lative effects  occur  and  that  in  some  cases  atomic  hydrogen  is 
present  in  sufficient  amount  to  show  its  own  critical  potentials. 
It  will  be  noticed  that  the  earlier  results  of  Davis  and  Goucher 
tend  to  support  the  view  of  Franck,  Knipping  and  Kriiger  rather 
than  the  other.  The  distinction  between  the  two  views  as  to  the 
interpretation  of  the  experimental  results  is  of  fundamental  im- 
portance in  the  theory  of  the  hydrogen  molecule.  Even  if  we 
restrict  ourselves  to  the  results  published  during  the  last  two 
years,  it  will  be  seen  that  there  is  lack  of  agreement  as  to  the  critical 
potentials  of  hydrogen,  and  still  more,  as  to  their  interpretation. 

While  Bohr's  theory  gives  a  wonderfully  accurate  picture  of  the 
hydrogen  atom,  it  gives  an  inexact  picture  of  the  hydrogen  mole- 
cule. The  calculated  work  of  dissociation  corresponds  to  2.70 
volts,  while  Langmuir's  experimental  work  yields  3.6  volts.  On 
Bohr's  theory,  an  impact  between  an  electron  and  a  hydrogen 
molecule  will  result  in  the  formation  of  a  neutral  atom  and  a 
charged  atom,  and  not  in  the  formation  of  a  positively  charged 

1  P.  R.,  17,  45  (1921). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


143 


molecule,  but  the  evidence  discussed  in  the  last  paragraph  shows 
that  it  is  produced.  Langmuir1  has  suggested  a  new  model  for 
the  hydrogen  molecule.  Each  electron  keeps  to  its  own  path, 
the  paths  being  arcs  of  curves  situated  in  a  plane  perpendicular 
to  the  line  joining  the  two  nuclei.  The  electron  in  the  path  ab 
always  keeps  the  same  distance  from  a,  as  the  other  electron  does  from 
a',  fig  22.  Langmuir  makes  use  of  the  known  work  of  dissociation 
to  calculate  the  magnitude  of  the  orbits  and  the  negative  energy 
of  the  system.  For  the  positively  charged  molecule,  he  assumes 
that  the  single  electron  oscillates  along  a  straight  line  perpendicular 
to  the  line  joining  the  nuclei,  fig.  23.  Two  ways  of  applying  the 


t 


Fir,.  22 


FIG   13. 


quantum  theory  are  suggested  to  determine  the  dimensions  of  the 
system .  On  taking  the  difference  between  the  energies  of  the  neutral 
molecule  and  of  the  charged  molecule,  Langmuir  obtained  10.15  and 
14.10  volts  as  the  ionizing  potential,  according  as  to  which  of  the 
two  ways  he  deduced  the  energy  of  the  charged  molecule.  The 
model  at  least  offers  a  possible  explanation  of  the  stable  charged 
molecule  and  of  an  ionizing  potential  near  to  1 1  volts. 

Helium.  According  to  Bohr,  the  negative  energy  of  the  helium 
atom,  pictured  as  a  system  of  two  electrons  rotating  around  a 
doubly  charged  nucleus,  is 

W  =  6.13  W0 

1  Science,  52,  433  (1920). 


144  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

The  negative  energy  of  the  positively  charged  helium  atom,  *.  e., 
with  only  one  electron  rotating  around  the  same  nucleus  is 

W  =  4.00  W0. 

This  positively  charged  atom  is  very  similar  to  the  hydrogen 
atom,  except  for  the  charge  on  the  nucleus,  and  should  give  out 
series  of  lines  represented  by 

4W0l 


Several  lines  (called  "enhanced"  lines)  belonging  to  these  series 
have  been  identified  by  Evans,  Paschen  and  Fowler  in  the  spectrum 
of  the  disruptive  discharge  through  helium,  where  we  may  suppose 
that  the  flow  of  current  is  so  intense,  momentarily,  that  many 
helium  atoms  ionized  by  one  impact  are  again  struck  before  they 
return  to  the  normal  state.  Compton  and  Lilly1  have  obtained 
the  enhanced  line  X  4868  in  an  intense  helium  arc. 

The  following  energies  are  required  to  effect  the  changes  indi- 
cated : 

A.  Neutral    atom     -  >     positively  volts 

charged  atom  ...............  .     6.  13  W0  —  4.00  W  =  2.  13  W0     (28.81) 

B.  Neutral      atom      -  >•      doubly 

charged  atom  ................     6.13W0—  0  =6.13W0     (82.90) 

C.  Charged      atom     -  >      doubly 

charged  atom  ................     4.00  W0  —  0  =  4.00  W0     (54.10) 

D.  Pos.  atom  -  >  pos.  atom  .......     4.00W0-  --  1    =  3.00  W0     (40.57) 


(State  1)  (State  2) 

(Giving  first  R.  P.  of  charged  atom) 


No  theoretical  value  for  the  radiating  potential  of  the  neutral 
helium  atom  has  been  given.  It  will  be  seen  that  the  experimental 
ionizing  potentials  for  the  helium  atom  are  around  25.5  volts, 
which  differs  from  the  theoretical  value  (28.86  volts)  by  more 
than  errors  of  observation. 

The  agreement  between  the  experimental  results  for  helium 
seems  to  be  better  than  in  the  case  of  other  gases.  Excepting 
the  values  of  20  volts  for  ionization,  which  can  easily  be  explained 
as  "cumulative"  effects,  as  will  be  seen  later,  it  appears  to  be  well 
established  that  the  ionizing  potential  for  the  normal  atom  is 
close  to  25.5  volts  and  the  radiating  potential  to  20.4  volts.  The 
former  value  is  definitely  lower  than  Bohr's  theoretical  values 
which  yield  28.81  volts,  by  about  3.3  volts.  Both  Horton  and 
Davies,  and  Franck  and  Knipping  found  that  a  second  type  of 

1  A.  P.  J.,  52,  1  (1920). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  145 

ionization  occurred  at  about  79.7  volts,  again  about  3.3  volts 
lower  than  Bohr's  theoretical  value  82.90  volts,  corresponding  to 
the  simultaneous  removal  of  both  electrons.  If,  however,  we 
calculate  the  ionizing  potential  for  a  charged  helium  atom,  i.  e., 
the  helium  nucleus  with  one  electron,  by  subtracting  the  first 
ionizing  potential  from  the  second,  we  get  according  to  Horton 
and  Davies  80.0  —  25.7  =  54.3  volts,  and  according  to  Franck 
and  Knipping,  79.5  —  25.4  =  54.1  volts.  These  values  are  ex- 
ceedingly close  to  the  theoretical  value  for  the  charged  helium 
atom,  viz.,  54.10  volts.  We  find  here,  therefore,  a  close  parallel 
to  the  case  of  hydrogen.  Bohr's  theory  is  quantitatively  exact 
when  we  have  to  deal  with  a  nucleus  and  one  electron,  whether 
that  nucleus  is  singly  or  doubly  charged  as  in  the  case  of  the  hydro- 
gen atom  and  the  (charged)  helium  atom,  respectively.  Bohr's 
model  is  not  quantitatively  exact  when  there  are  two  electrons 
outside  the  nucleus  to  be  considered  as  in  the  case  of  the  hydrogen 
molecule  and  the  normal  helium  atom.  By  a  skilful  variation 
in  the  choice  of  the  pressure  and  the  electron  current,  Horton  and 
Davies  were  able  to  show  in  the  same  apparatus  the  radiating 
and  ionizing  potentials  of  the  normal  helium  atom,  the  ionizing 
potential  corresponding  to  the  removal  of  both  electrons  from  the 
normal  atom,  and  the  radiating  and  ionizing  potentials  for  the 
charged  helium  atom.  The  latter  were  obtained  by  using  a  very 
intense  stream  of  electrons  so  that  there  was  opportunity  for 
electrons  to  hit  helium  atoms  already  ionized. 

Franck  and  Knipping1  discovered  the  existence  of  a  second 
radiating  potential  for  helium  by  means  of  an  ingenious  applica- 
tion of  the  Schuster-Rydberg  principle.  According  to  this  prin- 
ciple, the  difference  between  the  frequency  of  the  first  line  of  a 
series  and  the  frequency  of  the  limit  of  the  same  series  is  also 
the  frequency  of  the  limit  of  another  series.  Now  these  frequencies 
can  be  replaced  by  their  corresponding  critical  potentials.  The 
radiating  potential,  20.5  volts,  corresponds  to  the  first  line 
(IX  —  1Y)  of  a  series,  and  the  ionizing  potential,  25.4  volts, 
•corresponds  to  the  limit  (IX)  of  the  same  series.  (This  notation 
will  be  explained  in  a  later  section.)  Hence  the  difference  of  these 
values  should  correspond  to  the  limit  of  another  series. 

IX  —  1Y  =  20.5  volts 

IX  =  25.4  volts 

Hence  1Y  =    4.9  volts 

:'  P.  Z.,  20,487  (1919). 


146  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

The  frequency  of  the  limit  (1Y)  of  this  other  series,  corresponding 
to  4.9  volts,  should  be  X  2520.  Franck  and  Knipping  recognized 
that  this  was  close  to  the  limits  of  the  two  principal  series  for 
helium  X  2600  (4.74  volts)  and  X  3122  (3.95  volts),  respectively. 
(These  series  are  sometimes  referred  to  as  the  helium  and  parhelium 
series.)  The  natural  assumption  was  then  made  that  the  limit 
1Y  of  the  hypothetical  series  was  identical  with  the  known  limit, 
Is  (X  2600),  of  the  principal  series,  Is-mp,  for  helium.  The  exis- 
tence of  the  limit,  IS  (X  3122),  of  the  parhelium  series  suggested 
that  there  should  be  another  radiating  potential.  The  two  radiat- 
ing potentials  should  therefore  be 

25.4  —  4.74  =  20.66  volts 
25.4  —  3.95  =  21.45  volts 

i.  e.,  there  should  be  two  radiating  potentials,  separated  by  .8 
volt.  Careful  observations  on  pure  helium  showed  this  to  be  the 
case. 

In  a  second  paper  on  helium,  Franck  and  Knipping1  make  further 
important  contributions  to  our  knowledge  of  the  radiating  po- 
tentials of  helium.  They  start  with  the  view  that  in  the  normal 
helium  atom  the  two  electrons  cannot  exist  in  co-planar  orbits 
but  that  they  are  to  be  found  in  crossed  orbits,  i.  e.,  the  planes 
of  the  orbits  are  inclined  to  each  other  at  right  angles.  (Land6 
has  discussed  such  orbits  from  a  theoretical  standpoint.)  Thus 
the  only  possible  1 -quantum  state  for  the  helium  atom  is  that  in 
which  the  orbits  are  at  right  angles  (crossed  orbits).  The  next 
state,  the  2-quantum  state,  can  exist  in  two  forms,  i.  e.,  with  crossed 
orbits  and  with  co-planar  orbits.  Now  the  transition  from  the 
1 -quantum  state  to  the  2-quantum  state  requires  different  amounts 
of  energy  according  as  to  whether  the  second  state  is  that  of  crossed 
orbits,  or  that  of  co-planar  orbits.  According  to  Lande,  the  two 
possible  forms  of  the  second  state  (crossed  and  co-planar  orbits) 
for  the  atom  are  taken  to  account  for  the  two  principal  series 
(helium  and  parhelium).  Lines  of  helium  principal  series  (and 
other  series  having  the  same  limit)  are  given  out  when  an  electron 
falls  back  to  give  the  2-quantum,  co-planar  state,  and  lines  of  the 
parhelium  principal  series  (and  other  series  having  the  same  limit) 
are  given  out  on  return  to  the  2-quantum  crossed  state.  The 
two  radiating  potentials,  previously  found,  and  differing  by  .8 
volt,  correspond  to  displacements  of  an  electron  from  the  1 -quantum 

»Z./.  P.,  1,320(1920). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES         147 


state  to  the  2-quantum  crossed  state  and  to  the  2-quantum  co- 
planar  state,  respectively.  Fig.  24  represents  diagrammatically  the 
two  radiating  potentials  and  the  origin  of  the  helium  and  parhelium 
series. 

Franck  and  Knipping  investigated  the  radiating  potentials 
by  measuring  the  photo-electric  effect  of  the  radiation,  much  as 
in  the  Lenard  method.  The  electrons  were  accelerated  through 
a  gauze,  then  were  allowed  to  collide  with  molecules  in  a  com- 
paratively large  volume  bounded  by  a  second  gauze,  beyond  which 
was  the  photo-electric  plate.  The  most  important  result  of  this 
paper  is  that  in  the  very  purest  helium,  no  trace  of  the  20.45  volt 
radiating  potential  could  be  obtained.  In  such  a  case,  the  21.25 
volt  effect  shows  up  alone.  However,  the  slightest  amount  of 

/Vormai  Helium-  Atom  (u.u>u£M 
Statt,  State, 

1  Z 


-10' 


-4-0 -> 


FIG.  24. 

impurity  (such  as  can  be  obtained  by  warming  up  the  charcoal  on 
slightly  lowering  the  liquid  air  for  a  short  time)  causes  the  20.45 
volt  to  appear  again.  The  amount  of  impurity  was  considered  too 
small  to  show  any  ionization  or  radiation  by  itself.  It  is  suggested 
by  the  authors  that  the  2-quantum  co-planar  state  is  a  stable  state, 
and  that  the  helium  atom  will  not  spontaneously  return  from  it 
to  the  normal  1-quantum  state.  Thus  they  account  for  the  non- 
appearance  of  radiation  in  the  purest  helium  at  20.45  volts.  (It 
is  to  be  presumed  that  they  showed  that,  although  there  was  an 
absence  of  radiation,  there  were  inelastic  collisions  at  this  accelerat- 
ing voltage  in  the  purest  helium.)  The  2-quantum  co-planar 
helium  atom  resembles  the  hydrogen  atom  to  some  extent,  and 
may  possibly  be  capable  of  entering  into  transient  combination 
with  atoms  having  electron  affinities,  such  as  oxygen.  The  ap- 
pearance of  the  20.45  volt  radiating  potential  is  explained  on  the 


148 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


assumption  that  the  short-lived  compounds  formed  with  2-quan- 
tum  co-planar  helium  atoms  break  up  with  the  emission  of  radiation 
corresponding  to  20.45  volts.  The  impurities  thus  have  a  sort 
of  catalytic  effect  in  promoting  the  return  of  the  2-quantum  co- 
planar  helium  atom  to  the  normal  1 -quantum  state. 

The  idea  of  electrons  displaced  to  certain  orbits  outside  the  normal 
1 -quantum  orbit  being  unable  to  return,  and  so  giving  rise  to  a 
"metastable"  state  has  been  suggested  by  several  facts  in  spec- 
troscopy.  Thus  the  resonance  line  of  mercury  X  2536,  IS  —  Ipz, 
is  called  out  strongly  by  collisions  of  mercury  atoms  with  electrons 
having  energy  above  4.9  volts,  while  the  other  lines  of  the  triplet 
of  which  X  2536  is  the  middle  member,  are  so  feeble  that  they  have 
not  been  detected  until  recently.  It  would  appear  that  the  falling 
back  from  the  \p\  and  the  Ips  orbit  in  mercury  to  the  normal 
IS  orbit  does  not  occur  nearly  so  easily  as  from  the  lp%  orbit. 

The  conception  of  helium  atoms  in  a  metastable  state  makes 
the  explanation  of  ionization  below  the  ionizing  potential  by  the 
action  of  the  cumulative  effects  of  successive  collisions  much  more 
plausible  than  before  (see  later  section  on  cumulative  effects, 
particularly  the  part  dealing  with  Compton's  work  on  helium). 
The  absence  of  radiation  when  pure  helium  is  bombarded  by  20.45 
volt  electrons  and  presumably  being  changed  continuously  into 
the  metastable  state,  opens  up  interesting  possibilities  as  to  the 
physical  and  chemical  properties  of  this  new  type  of  helium. 

Franck  and  Knipping  found  several  discontinuities  in  their 
curves  showing  the  photo-electric  effect  of  radiation,  between 
the  first  radiating  potential  and  the  ionizing  potential.  They 
attributed  these  to  emission  lines  in  the  spectrum  of  helium.  The 
values  of  all  the  radiating  potentials  found  are  given  in  the  following 
table.  The  first  value  (20.45  volts)  is  enclosed  in  brackets  to 
show  that  it  does  not  appear  in  the  purest  helium. 


Observed 

X  from  h  v  =  Ve 

Calculated 

(20.45) 

21.25 
21.9 
23.6 

(610) 
585 
569 
523 

(20.45) 
21.25 
21.85 
23.7 

25.3 

493 

25.23 

A  series  formula  of  the  form 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  149 

N  N 

(i  +  *)»      (m  +  yY 

was  used  to  calculate  the  values  in  the  last  column.  The  constants 
were  determined  from  the  experimental  values  of  two  of  the  po- 
tentials, and  the  remaining  potentials  calculated.  It  will  be  seen 
that  the  results  are  in  good  agreement  with  the  formula. 

Professor  Lyman  in  a  letter  to  the  writer  states  that  he  has 
found,  spectroscopically,  a  strong  line  in  helium  at  X  585.  This 
value  corresponds  to  the  second  radiating  potential,  21.25  volts. 
No  line  appeared  to  correspond  to  the  commonly  found  radiating 
potential  at  20.45  volts.  It  is  unlikely  that  Lyman's  helium  (in 
a  vacuum  spectroscope)  could  be  so  free  from  impurities  as  that 
for  which  Franck  and  Knipping  found  the  20.45  volt  effect  to 
vanish.  Radiation  corresponding  to  this  at  about  X  605  might 
well  have  been  expected. 

Other  Gases.  As  we  have  no  satisfactory  atomic  models  of  the 
molecules  of  other  gases,  we  are  compelled  to  look  in  another — per- 
haps less  fundamental — direction  and  see  if  we  can  establish  some 
correlations.  On  the  analogy  of  satisfactory  correlations  in  the  case 
of  metallic  vapors,  attempts  have  been  made  to  apply  the  same 
results  to  gases.  Unfortunately  our  knowledge  of  the  spectrum 
in  the  extreme  ultraviolet,  and  especially  of  series  there,  is  not 
nearly  so  complete  as  in  the  region  X  7000-X  2000.  Smyth  and 
Mohler  and  Foote  identified  their  values  (8.29  and  8.18,  respectively) 
with  the  nitrogen  doublet  X  1492.8  and  X  1494.8,  which  would 
correspond  to  8.26  volts.  According  to  Lyman,  oxygen  has  an 
absorption  band  in  the  ultraviolet,  with  its  center  about  X  1400. 
This  expressed  in  volts  is  8.8  volts,  and  possibly  corresponds  to  the 
experimental  critical  potentials  about  9.0  volts.  No  other  corre- 
lation, outside  metallic  vapors,  appears  to  have  been  made.  Further 
work  on  the  emission  and  absorption  spectra  of  gases  and  vapors 
in  the  extreme  ultraviolet  is  much  to  be  desired  to  supply  data 
for  comparison  with  critical  potentials. 

In  this  section  we  may  refer  to  Compton  and  Smyth's  work1 
on  iodine.  They  found  two  ionizing  potentials,  in  iodine  vapor, 
one  corresponding  to  the  molecule  (10.0  volts)  and  the  other  to 
the  atom  (8.5  volts).  As  the  temperature  is  raised,  and  the  amount 
of  dissociation  increased,  the  intensity  of  the  8.5  volt  effect  increases 
with  respect  to  the  10.0  volt  effect,  as  would  be  expected.  The 

1  Science,  51,  571  (1920);  P.  R.,  16,  501  (1920). 


150  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

difference  corresponds  to  the  value  of  the  work  necessary  to  dis- 
sociate iodine  as  determined  by  the  methods  of  physical  chemistry. 

SPECTRAL  SERIES  NOTATION* 

The  spectra  of  many  metals,  particularly  those  of  the  alkali 
metals  and  of  the  alkaline  earths,  can  be  resolved  into  a  number 
of  series.  The  frequencies  of  lines  in  a  series  of  lines  in  the  spectrum 
of  an  element  may  be  represented  as  the  difference  of  two  terms, 
such  as 

N  N 


which  represents  the  Principal  Series.  To  a  first  approximation, 
the  functions  are  such  that  the  difference  may  be  written 

N  N 

~  (1  +  S)2        (m  +  P)2 

"N"  is  a  universal  constant.  "S"  and  "P"  are  constants  for  this 
particular  series,  and  m  takes  successive  integral  values  up  to 
infinity,  each  value  corresponding  to  a  line  in  the  series.  It  will 
be  seen  that  as  m  increases  indefinitely,  the  frequencies  become 
closer  and  closer  together,  and  when  m  =  <»  ,  we  have  only  the 
first  term  left  which  is  called  the  "convergence  frequency"  or 
"limit"  of  the  series.  The  first  line  of  the  series  is  usually  that 
for  which  m  is  the  smallest  integer  which  will  make  the  expression 
positive.  For  example,  the  first  line  of  the  Principal  Series  is 

N  N 

~  (1  +  S)2       (1  +  P)2 
A  short  notation  for  this  series  is 

v  =  IS  —  mP 
The  chief  series  are  as  follows: 


Principal  IS  —  mP 

Sharp  IP  — mS 

Diffuse  IP  —  wD 

Fundamental  2D  —  wF 


m  =  1,2,  3,  4. 
m  =  2,  3,  4,  5. 
m  =  2,3,  4,  5. 


m  =  2,  3,  4,  5. 


*  The  author  wishes  to  thank  Professors  A.  Fowler  and  F.  A.  Saunders  for  valu 
able  information  in  this  connection.  The  author  has  adopted  their  notation,  which 
is  that  generally  used  by  American  and  English  spectroscopists,  in  preference  to  that 
often  followed  by  writers  on  ionizing  and  radiating  potentials,  as  it  seems  desirable 
for  the  sake  of  uniformity  that  all  should  follow  the  same  notation.  The  notation 
will  be  used  by  Professor  Fowler  in  his  forthcoming  book  "Report  on  Series  Spectra" 
to  be  published  by  the  London  Physical  Society.  Much  information  on  series  notation 
-.ill  be  found  in  papers  by  Professor  Saunders  (.4.  P.  J.,  41,  323  (1915);  50,  151  (1919); 
51,  23  (1920)). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  151 

"Combination"  series  are  also  found,  formed  by  taking  terms 
from  two  different  series  of  those  mentioned  above;  e.  g.,  IS  —  wD 
is  a  combination  series. 

The  radiating  and  ionizing  potentials  of  the  alkali  metals  are 
found  to  be  related  to  the  "principal  series  of  doublets,"  for  which 
the  above  notation  is  used,  except  that  Greek  letters  are  used 
instead  of  capitals.  When  it  is  necessary  to  distinguish  between 
the  two  lines  of  a  doublet,  suffixes  are  used.  Thus  the  two  lines 
forming  the  familiar  doublet  of  sodium  X  5890  and  X  5896  are 
la-  —  l7rb  and  Iff  —  l7T2,  respectively. 

When  we  deal  with  the  radiating  and  ionizing  potentials  of  the 
metals  of  the  second  column  in  the  periodic  table,  we  have  to  do 
with  the  "principal  series  of  single  lines"  or  "singlets,"  as  they 
are  frequently  called,  IS  —  mP,  and  also  with  a  combination 
series  formed  by  subtracting  from  the  convergence  frequency 
of  the  principal  series  of  singlets  IS,  the  terms  mpz  of  the  principal 
series  of  triplets  (middle  terms  only).  The  combination  series 
referred  to  is  IS  —  mpz.  Thus  the  first  line  of  the  principal  series 
of  singlets  IS  — •  mP  (m  =  1)  for  mercury  is  X  1849,  the  first  line 
of  the  combination  series  IS  —  mpz  (m  —  1)  is  X  2537,  while 
the  limit  for  both  series,  IS,  is  at  X  1188. 

A  method  of  visualizing  the  origin  of  series  in  the  Hg  spectrum 
is  shown  in  fig.  25.  The  various  vertical  lines  IS,  IP,  2S,  etc., 
and  lplt  lpz,  Ipa,  etc.,  represent  some  of  the  possible  stationary 
states  of  the  atom,  in  Bohr's  sense.  We  have  no  means  of  know- 
ing the  actual  spacing  between  the  orbits,  or  how  they  are  arranged 
in  such  a  complex  .system  as  the  Hg  atom,  but  we  do  know  the 
wave-length  of  the  lines  given  out  as  an  electron  falls  back  from 
one  of  the  outer  stationary  orbits  to  one  of  the  inner  orbits.  Since 
the  emission  of  lines  is  supposed  to  be  governed  by  the  quantum 
relation  Ve  =  hv,  we  know  the  energy  emitted  by  the  atom  for 
each  line,  and  hence  the  diagram  is  plotted  in  terms  of  energy 
(expressed  in  volts)  below  and  in  the  equivalent  wave  numbers 
(=  i/\  =  v/3(10)10)  above.  The  diagram  therefore  shows  the 
energy  of  the  atomic  system  according  to  which  of  the  stationary- 
orbits  is  occupied  by  the  outermost  electron,  and  the  wave  num- 
ber of  the  line  emitted  as  the  electron  falls  from  an  outer  orbit 
to  an  inner  orbit.  (Possibly  it  would  be  more  correct  to  say  that 
the  vertical  lines  specify  the  energy  of  the  various  possible  con- 
figurations of  the  atom,  and  that  it  is  conjectured  that  these  con- 


152  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

figurations  correspond  to  the  atom  with  its  outermost  electron  in 
one  or  other  of  the  possible  stationary  orbits.  The  energy  re- 
lations are  definite,  the  way  in  which  they  are  accounted  for  is 
still  to  be  settled.)  Thus  the  energy  given  out  by  the  atom  when 
the  line  X  2536  is  emitted  is  measured  by  4.9  volts,  and  the  energy 
given  out  when  the  electron  is  completely  removed  is  10.4  volts. 
In  the  normal  unexcited  atom,  the  orbits  outside  the  IS  orbit 
are  supposed  to  be  unoccupied.  Evidence  for  this  will  be  given 
later.  In  the  upper  part  of  the  diagram  will  be  found  a  few  of  the 
spectral  lines  (shown  by  horizontal  lines)  in  three  singlet  series, 
viz.,  the  Principal,  IS— mP,  the  Sharp  IP— mS,  and  the  Diffuse 
IP  — mD.  In  the  lower  part,  one  series  of  triplets,  Ip— ms,  is  illus- 
trated. In  the  middle,  several  lines  belonging  to  combination 
series  are  shown,  e.  g.,  IS—mp.  Fig.  25  is  shown  to  illustrate 
Franck  and  Einsporn's  paper  which  will  be  considered  later.  The 
full  horizontal  lines  are  those  which  they  identified  through  radiat- 
ing potentials.  For  the  present  discussion,  the  distinction  be- 
tween full  and  dotted  horizontal  lines  is  to  be  ignored. 

The  following  table  shows  the  correspondence  between  the 
notation  used  by  Ritz  and  Paschen  and  that  used  here,  following 
Fowler  and  Saunders. 


Ritz  and  Paschen 

Fowler  and  Saunders 

1  5S  2  5S  3  5S 

IS    2S    3S 

2P,  3P 

IP    2P 

- 

3D,  4D  

2D  3D 

1  .  5S  —  mP,     m  =  2,3  

IS  —  mP,.  m 

=  1,2  

2P  —  mS,         m  =  2.5,3.5. 

IP  —  mS,  m 

=  2,3  

2P  —  wD,       m  =  3,4  

IP  —  mD,  m 

=  2,3  

1  .  5S  —  mp2,    m  =  2,3  IS  —  m/>2,  m 

=  1,2  

Metallic  Vapors.  It  was  noticed  by  Davis  and  Goucher  that  in 
the  case  of  mercury  vapor  there  were  radiating  potentials  corre- 
sponding to  the  lines  X  2536  and  X  1849  and  an  ionizing  potential 
corresponding  to  X  1188,  the  limit  of  both  the  series  to  which  X  2536 
and  X  1849  belong.  This  gave  the  clue  to  another  method  of  deter- 
mining the  ionizing  potentials  and  was  used  very  successfully  by  Mc- 
Lennan and  Young.1  It  was  assumed  that  any  method  which 
would  give  the  convergence  frequency  of  the  principal  series  would 
at  the  same  time  give  the  ionizing  potential  through  the  quantum 

1  P.  R.  S.,  95,  273  (1918). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  153 

relation.  McLennan  and  Young  made  use  of  the  property  that 
lines  in  the  principal  series  are  strongly  marked  absorption  lines 
when  light  is  passed  through  the  vapor  of  the  element.  In  this 
way  they  were  able  to  pick  out  the  lines  in  the  principal  series 
of  a  number  of  elements,  and  so  to  calculate  the  convergence 
frequency  and  the  ionizing  potential.  They  gave  the  following 
values  of  the  ionizing  potentials  calculated  by  this  method : 


Hg  10.  45  volts 

Zn  9.4    volts 

Cd  9.0    volts 

Mg  7.  65  volts 

Ca  6.  12  volts 


Sn  5.  70  volts 

Ba  5.21  volts 

Na  5.  13  volts 

Ka  4.  32  volts 


Mohler  and  Foote,  and  others,  have  made  extensive  deter- 
minations of  radiating  and  ionizing  potentials  of  a  large  num- 
ber of  metals.  They  conclude  that  in  the  case  of  the  elements 
Na,  K,  Rb  and  Cs  (Group  I  of  the  Periodic  Table)  the  ionizing 
potential  corresponds  to  la  and  the  radiating  potential  to  the 
shorter  line  l<r  —  l7r2  of  the  doublet,  l<r  —  ITT,  in  the  principal 
series  of  doublets.  For  the  elements  Mg  and  Ca,  Hg,  Zn,  Cd, 
they  found  that  the  radiating  potential  corresponds  to  the  first 
line  of  the  combination  series  IS  —  l£>2  and  the  ionizing  potential 
to  IS.  There  is,  however,  in  Ca  a  radiating  potential  at  IS  —  IP 
as  well,  and  possibly  a  corresponding  one  would  be  found  for  Mg. 
Davis  and  Goucher  have  shown  that  a  second  radiating  potential 
exists  for  Hg  corresponding  to  IS  —  IP.  It  seems  safe  to  generalize 
that  IS  corresponds  to  the  ionizing  potentials  of  the  elements  in 
this  group,  and  that  IS  —  Ipz  and  IS  —  IP  correspond  to  radiat- 
ing potentials,  although  the  latter  may  often  be  masked  by  the 
former.  * 

Not  enough  is  known  about  series  in  the  spectra  of  ele- 
ments outside  the  first  two  groups.  Mohler  and  Foote  have 
indeed  suggested  that  the  experimental  values  of  the  ionizing  and 
radiating  potentials  for  such  elements  may  be  used  as  starting 
points  in  the  search  for  series  relations. 

SINGLE  AND  MULTIPLE  LINE  SPECTRA 
The  work  of  Franck  and  Hertz1  in  1914  must  be  regarded  as  the 

*  In  a  recent  paper,  Foote.  Mohler  and  Meggers  (Bur.  Stan.,  1920,  725)  find  that 
every  element  in  Group  II,  which  they  tried,  viz.,  Zn,  Cd,  Hg,  Mg  and  Ca,  has  two 
radiating  potentials,  corresponding  to  IS  —  Ipt  and  IS  —  IP. 

1  V.  d.  D.  P.  G.,  16,  512  (1914). 


154  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

starting  point  of  all  work  on  radiating  potentials,  for  they  were 
the  first  to  demonstrate  that  by  bombarding  the  atoms  of  mercury 
vapor  with  electrons,  the  single  line  X  2536  appeared  as  soon  as 
the  energy  of  the  electrons  exceeded  4.9  volts.  This  was  the  first 
proof  of  the  quantum  relation  as  applied  to  the  direct  production 
of  radiation  by  electron  impacts,  and,  moreover,  served  to  identify 
the  radiation  associated  with  the  radiating  potential  as  mono- 
chromatic light  whose  wave-length  is  that  of  the  first  line  in  an 
important  series  of  the  spectrum.  McLennan  and  Henderson1 
and  McLennan2  extended  this  investigation  to  other  metals. 
It  was  found  that  for  Hg,  Zn,  Cd  and  Mg,  electrons  must  possess 
a  certain  characteristic  minimum  velocity  before  the  single  lined 
spectrum  of  these  elements  could  be  called  out,  and  that  this 
minimum  velocity  agreed  well  with  the  value  deduced  by  the 
quantum  relation  from  the  corresponding  wave-lengths.  These 
wave-lengths  X  2536  for  Hg,  X  3076  for  Zn,  X  3260  for  Cd,  and 
X  2852  for  Mg  are  all  first  lines  of  important  series.  The  first 
three  lines  belong  to  the  combination  series,  being  IS  —  lpz,  while 
the  Mg  line  is  the  IS  —  IP  line.  It  should  be  noted  here  that 
Mohler  and  Foote  in  their  experiments  on  radiating  potentials 
found  that  the  frequency  IS  —  lpz  could  be  excited  with  Mg 
just  as  with  the  other  elements.  They  state  that  their  method 
really  measures  the  points  of  inelastic  impact  hitherto  taken  to 
indicate  the  wave-length  of  the  principal  radiation.  For  Mg  and 
Ca  (and  presumably  Ba  and  Sr)  the  inelastic  collision  method 
emphasizes  the  frequencies  IS  —  Ifa  and  indicates  the  presence 
of  IS  —  IP  while  the  spectroscopic  evidence  emphasizes  IS  —  IP. 
McLennan  and  Henderson  found  that  as  the  speed  of  the  elec- 
tron was  increased  nothing  but  the  single  line  was  obtained  until 
the  speed  corresponding  to  the  head  of  the  series  was  reached,  at 
which  point  all  the  lines  of  the  series  appeared  together.  It 
would  seem  natural  to  expect  that  shorter  lines  than  the  single 
lines  would  appear  when  electrons  of  the  proper  velocity  were 
driven  through  the  vapor.  McLennan  and  Ireton3  investigated 
this  carefully  and  found  that  in  the  case  of  Zn  and  Cd  two  lines, 
and  only  two  lines,  could  be  called  out  by  electrons  having  just 
sufficient  energy  to  call  out  the  shorter.  (If  the  conditions  are 
not  such  as  to  render  cumulative  effects  negligible,  other  lines  may 

1  P.  R.  S.,  91,  485  (1915). 

2  Ibid.,  92,  305  (1916). 
'  P.  M.,  36,  46  (1918). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  155 

appear  as  well.)  The  first  line  called  out  in  each  case  is  the  first 
line  of  the  combination  series  IS  —  Ifa  and  the  second  line  is  the 
first  line  of  the  principal  series  IS  —  IP. 

Foote  and  Meggers1  have  made  some  very  interesting  investi- 
gations on  the  spectrum  of  Cs  excited  by  slow  electrons.  Measure- 
ments of  the  energy  in  the  various  spectral  lines  were  made  for 
different  values  of  the  accelerating  potential.  It  was  concluded 
that  the  two  lines  of  the  doublet  la  —  ITT  were  called  out  simulta- 
neously, and  that  they  were  called  out  only  when  the  electrons 
had  energy  in  excess  of  that  corresponding  to  the  shorter  of  the 
two  lines  forming  the  doublet.  No  other  lines  were  called  out 
until  the  ionizing  potential  was  reached.  This  result  seems  to  be 
in  conflict  with  that  of  Franck  and  Knipping  on  helium,  and 
with  that  of  Franck  and  Einsporn  on  mercury.  There  seems  to 
be  no  a  priori  reason  for  supposing  that  the  lines  of  higher  fre- 
quencies in  either  the  IS- IP  or  the  IS—  \p  series  cannot  be  ex- 
cited, with  electrons  of  appropriate  energy,  if  the  first  lines  can  be 
excited.  One  would  expect  the  mechanism  to  be  the  same  for 
all  lines  of  the  series,  the  only  difference  being  the  amount  of  energy 
involved.  Possibly  the  intensities  of  the  lines  -of  higher  fre- 
quencies are  too  feeble,  under  most  experimental  conditions,  to 
allow  them  to  be  detected. 

Franck  and  Einsporn2  have  recently  made  a  very  important 
contribution  to  our  knowledge  of  the  radiating  potentials  of  mer- 
cury. They  measured  the  photo-electric  effect  produced  by  elec- 
tron impacts  with  mercury  atoms,  as  a  function  of  the  potential 
accelerating  the  electrons.  By  taking  great  care  to  use  pure 
mercury  vapor,  to  secure  absolutely  steady  electron  emission 
from  the  source  and  to  increase  the  accelerating  potential  by  very 
small  steps,  they  found  no  less  than  eighteen  discontinuities  in 
their  curves  between  the  first  radiating  potential  and  the  ionizing 
potential.  (They  used  a  modified  Lenard  method.)  The  results 
are  given  in  the  table.  They  were  able  to  identify  a  number  of 
the  discontinuities  with  known  lines.  In  some  cases,  e.  g.,  No.  17 
and  17',  the  theoretical  values  corresponding  to  possible  interpreta- 
tions of  the  breaks  in  the  curves  are  so  close  that  it  is  not 
possible  to  decide  which  interpretation  is  correct.  Corresponding 
to  No.  3  there  is  the  known  optically  absorbing  region  from 
X  2313  to  X  2338,  which  does  not  fit  into  any  known  series  rela- 

1  P.  M.,  40,  80  (1920). 
2Z./.  P.,  2,  18(1920). 


156 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


tions.  There  are  some  breaks,  e.  g.,  6,  7,  9,  ....  which  do  not 
correspond  to  any  spectrum  lines  known  at  present.  The  break, 
No.  5,  is  of  interest  in  that  Franck  and  Einsporn  suggest  that 
it  is  due  to  the  complete  expulsion  of  an  electron  from  the 
orbit  Ip3,  i.  e.,  to  ionization  of  an  abnormal  Hg  atom,  abnormal 
in  the  sense  that  in  the  normal  Hg  atom,  no  electrons  exist  outside 
the  orbit  IS.  It  is  well  known  that  in  the  triplet  IS  —  Ip  the 


No. 

Observed 
V. 

Strength 
s*»strong,  «  =  mediuin, 

X 

Series 

Calculated 
V. 

1 

4.68 

W 

2656  .  5 

IS  —  1ft 

4.66 

2 

4.9 

very  s,  especially  at 

2537 

IS  —  1ft 

4.86 

high  pressures 

2338 

5.28 

3 

5.32 

w 

2331 

5.34 

4 

5.47 

}  w  at  mean  pressures 

If 

2270.6 

IS  —  1ft 

5.43 

5 

5.76 

s 

2150 

1ft 

5.73 

6 

6.04 

w 

2043 

7 

6.30 

w 

(1956) 

8 

6.73 

m 

1849-6 

IS—  IP 

6.67 

9 

7.12 

/  s  high  pressures 
\  w  low  pressures 

(1733) 

10 

7.46 

m 

1656 

11 

7.72 

m 

1603.9 

IS—  Is 

7.69 

12 

8.35 

IV 

(1447) 

13 

8.64 

s 

1435.6 

IS  —  2ft 

8.58 

14   \ 

(  m 

1402.7 

IS  —  2P 

8.79 

14'  } 

8.86 

\m 

1400 

IS  —  2d 

8.81 

15 

907 

2656.5 

2  X  4.66 

.  o/ 

w 

2656.5 

=  9.32 

16 

w 

1307.8 

IS  —  3ft 

9.44 

«4 

9.60 

w 

2656.5 

4.66  +  4.86 

J 

2537 

-  9.52 

17 

9.7 

m 

2537 

2  X  4.86 

17'  1 

2537 

IS  —  3P 

=  9.72 

J 

m 

1268.9 

9.73 

18 

10.38 

s  low  pressures 

1187.9 

IS 

10.39 

w  high  pressures 

middle  term  IS  —  Ip?  is  far  more  intense  than  either  IS  —  \p\ 
or  IS  —  \p%.  The  electrons  apparently  have  much  more  difficulty 
in  dropping  back  from  orbits  I  pi  or  Ip3  to  IS  than  from  lpt  to  IS, 
consequently  it  has  been  suggested  that  a  mercury  atom  with 
electrons  in  these  orbits  is  in  a  semi-stable  state.  Hence  there  is  a 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


L57 


far  greater  probability  of  impinging  electrons  hitting  an  atom  in 
these  states,  and  so  causing  ionization  or  radiation,  of  another- 
type,  than  for  the  case  of  an  atom  with  an  electron  in  the  orbit 
Ipz.  The  diagram  (based  on  those  given  in  Franck  and  Einsporn's 
paper)  shows  some  of  the  possible  orbits  (in  Bohr's  sense)  in  the 
mercury  atom  (fig.  25).  They  are  so  spaced  as  to  measure  the 


FIG.  25. 


frequency  difference  between  each  orbit  and  consequently  the 
energy  necessary  to  shift  an  electron  from  any  orbit  to  any  other. 
The  left  boundary  of  the  diagram  is  in  the  IS  orbit,  the  outermost 
orbit  occupied  by  an  electron  in  the  normal  atom.  The  right 
boundary  corresponds  to  an  orbit  at  infinity,  and  the  distance 


158  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

between  the  boundaries  measures  the  ionizing  potential.  The 
horizontal  lines  (whose  length  is  a  wave  number)  correspond  to  spec- 
tral lines  denoted  by  the  position  of  their  ends.  Thus  four  lines 
in  the  principal  series  IS  —  mP  are  shown.  The  full  lines  shown 
in  the  diagram  are  those  which  probably  correspond  to  the  radiating 
potentials  found  by  Franck  and  Einsporn.  (The  dotted  lines 
illustrate  other  lines  belonging  to  a  few  of  the  well-known  series  of 
mercury.) 

CUMULATIVE  EFFECTS — LOW  VOLTAGE  ARCS 

In  this  section  will  be  considered  a  group  of  investigations  on 
the  phenomena  which  only  become  evident  when  sufficiently 
dense  electron  streams  are  passed  through  a  gas,  and  whose  density 
is  usually  appreciable.  In  the  foregoing  sections,  attention  has 
chiefly  been  paid  to  those  results  which  bear  out  the  quantum 
relations.  But  certain  investigations  have  been  published,  which, 
at  first  sight,  seem  in  conflict  with  the  results  expected  from  theory, 
and  confirmed  by  other  researches.  In  general,  these  effects 
come  into  play  only  when  the  impact  of  an  electron  on  a  molecule 
can  no  longer  be  regarded  as  an  isolated  event,  that  is,  when  the 
result  of  an  impact  is  not  influenced  by  the  radiation  to  which  the 
molecule  is  exposed  from  other  impacts  in  its  vicinity,  and  by 
impacts  which  it  may  have  experienced  a  short  time  previously. 

As  the  supply  of  electrons  is  increased  when  electrons  are  driven 
through  a  gas  at  a  low  voltage  (provided  pressure  and  voltage  are 
suitable)  a  more  or  less  visible  discharge  (an  arc)  may  suddenly 
set  in,  a  large  increase  in  the  current  occurring  simultaneously. 
This  increase  in  current  indicates  ionization.  It  became  evident, 
however,  that  a  voltage  much  less  than  the  ionizing  potential  would 
produce  an  arc  under  favorable  conditions,  thus  throwing  doubt 
on  whether  any  fundamental  importance  could  be  attached  to  the 
so-called  ionizing  potentials. 

Richardson  and  Bazzoni1  found  that  with  an  intense  electron 
stream  through  helium,  an  arc  could  be  made  to  strike  when  the 
potential  accelerating  the  electrons  was  as  low  as  22.5  volts  and 
mentioned  that  there  were  indications  that  the  striking  potential 
tended  to  still  smaller  values.  Hebb2  found  that  a  mercury  arc 
could  be  started  with  electrons  whose  velocities  correspond  to  4.7 
volts  only,  and  that  it  could  be  maintained  at  3.2  volts.  Later,3 

1  Nature,  48,  5  (1916). 

2  P.  R.,  9,  371  (1917). 
» Ibid.,  ii,  170  (1918X. 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  159 

he  maintained  that  Hg  was  directly  ionized  by  electrons  of  4.9 
volts  velocity  and  that  no  explanation  of  the  effect  as  a  secondary 
phenomenon  could  be  considered  adequate.  In  another  research1 
on  arcs  in  mixtures  of  K  and  Hg  and  of  Na  and  Hg,  he  found  that 
an  arc  could  be  struck  in  the  former  at  1.6  volts  and  in  the  latter 
at  2.5  volts  and  maintained  in  them  at  .5  and  1.4  volts,  respectively. 
In  the  case  of  the  arc  in  the  mixture  of  Hg  and  K  vapors,  it  is 
interesting  to  note  that  even  when  the  electrons  had  a  velocity 
of  only  .5  volt,  the  mercury  spectrum  was  visible.  This  value, 
.5  volt,  as  well  as  the  1.4  volt,  at  which  the  arc  was  started,  is, 
of  course,  much  below  the  ionizing  and  radiating  potentials  of  Hg. 
Very  recently  Hebb2  concluded  that  ionization  of  Hg  vapor  can 
take  place  at  potentials  as  low  as  3.2  volts,  with  very  intense 
electron  streams,  and  that  the  previous  limit  given  for  ionization, 
viz.,  4.9  volts,  has  no  special  significance.  Tate3  showed  that 
with  large  electron  streams  through  Hg  vapor,  arcs  could  be  started 
with  velocities  down  to  7.3  volts.  McLennan4  found  that  the 
potentials  at  which  arcs  would  strike  depended  very  much  on  the 
density  of  the  electron  stream,  and  the  density  of  the  vapor,  de- 
creasing as  these  factors  increased.  With  Hg  and  Cd,  the  lowest 
potentials,  under  the  most  favorable  conditions  at  which  arcs 
could  be  struck,  were  4.75  and  5  volts,  respectively,  while  they 
could  be  maintained  at  potentials  as  low  as  2.84  and  2.0  volts. 
It  was  recorded  that  close  to  these  minimum  striking  potentials 
the  arc  took  an  appreciable  time  to  start,  after  the  potential  was 
applied.  It  should  be  noted  that  time  lag  effects,  of  which  this 
is  one,  have  frequently  been  observed  in  connection  with  phe- 
nomena associated  with  intense  electron  currents  through  gases.5 

As  will  be  seen  from  the  table,  several  investigators  have  found 
strong  ionization  in  helium  and  argon  at  potentials  at  which  other 
experimenters  find  radiation  only.  (In  the  experiments  showing 
ionization,  dense  electron  currents  and  appreciable  pressures  were 
used.)  Compton,  Olmstead,  and  Lilly6  found  that  with  a  suffi- 
ciently dense  stream  of  electrons  in  He  at  a  suitable  pressure,  arcs 
could  be  started  at  a  potential  as  low  as  20.2  volts,  i.  e.t  the  radiating 

ip.  R.,  12,486  (1918). 
*Ibid.,  15,  130  (1920). 
3  Ibid.,  io,81  (1917). 
*  P.L.P.  5.,  31,  1  (1918). 

5  Richardson  and  Bazzoni,  P.  M.,  32,  426  (1916);  Tate,  P.  R.,  10,  81  (1917);  Comp- 
ton, Lilly  and  Olmstead,  Ibid..  15,  ->ir>  (1920). 

6  P.  R.,  15,  545  (1920). 


160  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

potential,  and  could  be  maintained  at  still  lowe'r  potentials,  8  volts 
being  the  lowest.  However,  no  further  decrease  in  the  striking 
potential  (20.2  volts)  could  be  obtained  by  any  further  increase  in 
the  density  of  the  electron  stream  or  change  in  pressure.  It  would 
appear  that  these  experiments  definitely  prove  that  the  radiating 
potential  in  helium  is  the  lowest  potential  at  which  ionization  can 
be  started,  even  under  the  most  favorable  conditions,  a  result 
which  is  in  agreement  with  McLennan's  results,  and  with  Hebb's 
earlier  results. 

Various  explanations  of  these  apparently  anomalous  effects 
with  dense  electron  streams  through  gases  at  appreciable  pres- 
sures have  been  offered.  Richardson  and  Bazzoni1  suggest  that 
ionization  at  potentials  lower  than  those  corresponding  to  the 
theoretical  ionizing  potential  is  the  result  of  successive  impacts, 
or  results  from  impacts  on  atoms  in  an  abnormal  condition  caused 
by  the  absorption  of  radiation  generated  in  other  atoms. 

Millikan2  suggested  that  just  above  the  radiating  potential 
(this  term  was  not  in  use  at  the  time),  4.9  volts  for  Hg,  the  radia- 
tion (X  2536)  produced  might  act  photo-electrically  on  the  neigh- 
boring atoms,  certainly  those  of  the  cathode  and  possibly  those 
of  the  surrounding  vapor,  and  thus  add  more  electrons  to  the 
original  stream.  The  electrons  so  produced  would  be  accelerated 
by  the  field  and  might  produce,  if  favorably  placed,  more  radiation, 
which,  in  turn,  would  release  more  electrons  photo-electrically. 
In  this  way,  it  could  be  readily  seen  how  an  indefinite  multiplica- 
tion of  the  original  electron  current  might  occur,  and  how  ioniza- 
tion would  accompany  the  radiation  produced  at  4.9  volts.  The 
most  important  feature  of  Millikan's  theory  is  that  charged  atoms 
resulting  from  the  expulsion  of  the  photo-electrons  would  permit 
(1)  ionization,  (2)  the  many-lined  spectrum,  and  (3)  the  low  voltage 
arc,  all  below  the  accepted  ionizing  potential  10.4  volts.  This 
follows  from  the  fact  that  an  atom  which  had  lost  an  electron 
photo-electrically  would  sometime  later  regain  an  electron,  and 
this  would  pass  from  one  Bohr  orbit  to  another  on  its  way  to  the 
orbit  which  it  would  finally  occupy  in  the  normal  atom.  In  jump- 
ing from  one  orbit  to  another  radiation  of  the  corresponding  wave- 
length would  be  given  out,  and  thus  the  production  of  the  many- 
lined  spectrum  could  easily  be  accounted  for.  Also,  it  would  need 
but  very  little  energy  on  the  part  of  an  impinging  electron  to 

1  Nature,  48, 5  (1916). 
3  P.  R.,  9,  378  (1917). 


REPORT  0.\  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  161 

ionize  an  atom  in  which  the  electron  happened  to  be  in  one  of 
the  outer  orbits  while  on  its  way  inwards  to  the  orbit  normally 
occupied.  Thus  ionization  of  abnormal  atoms  was  accounted  for — 
hence  low  voltage  arcs.  Atoms  in  all  stages  of  recovery  to  normal 
would  be  found,  and  the  effects  (1),  (2)  and  (3)  would  be  weak  or 
strong  according  as  few  or  many  of  the  outer  orbits  were  occupied 
by  electrons.  The  minimum  striking  potential  for  the  arc  (4.9 
volts)  was  taken  to  show  that  the  outermost  orbit  occupied  in  the 
normal  Hg  atom  was  the  one  corresponding  to  the  limit  of  the 
series  of  which  X  2536  is  the  first  member,  for,  unless  the  impinging 
electron  has  sufficient  energy  to  displace  an  electron  to  the  next 
possible  orbit,  no  radiation  (and  ionization  resulting  therefrom) 
can  possibly  occur.  Though  it  is  now  evident  that  X  2536  cannot 
ionize  Hg  vapor,  Millikan's  theory  would  still  hold  if,  for  the 
complete  ionization  of  the  Hg  atom  photo-electrically,  one  sub- 
stituted partial  ionization,  i.  e.,  the  displacement  of  the  electron 
from  the  normal  orbit  IS  to  the  l£2  orbit,  in  which  state  the  atom 
could  be  ionized  by  subsequent  impacts  of  electrons  with  energy 
well  below  10.4  volts. 

Van  der  Bijl1  suggested  that  the  production  of  ionization,  the 
many-lined  spectrum,  and  arcs  with  potentials  below  the  ionizing 
potential,  could  be  accounted  for  on  the  assumption  that  with 
dense  electron  streams  an  atom  would  frequently  experience  a 
second  collision  while  still  in  an  abnormal  state  resulting  from  a 
previous  collision.  Thus  if  we  have  a  stream  of  electrons  with 
energy  corresponding  to  5  or  6  volts  passing  through  Hg  vapor, 
the  first  collision  with  an  atom  will  displace  an  electron  from  the 
normal  orbit  to  the  orbit  Ifa.  The  atom  is  now  in  a  state  that 
needs  but  10.4  —  4.9  =  5.5  volts  to  remove  the  electron,  so  that  a 
second  collision  may  effect  complete  ionization.  The  idea  here 
involved  is  frequently  referred  to  as  that  of  successive  impacts. 

McLennan2  attributed  the  production  of  "faint  arcs"  showing 
the  many-lined  spectrum,  at  potentials  much  below  the  ionizing 
potential,  to  the  presence  of  electrons  of  abnormal  velocities  among 
those  emitted  by  the  incandescent  source.  In  one  case,  electrons 
of  initial  energy  corresponding  to  5  volts  would  have  to  be  assumed. 
For  these  arcs,  it  was  suggested  that  the  direct  action  of  the  few  high 
speed  electrons  would  be  sufficient.  (A  calculation  of  the  proportion 
of  electrons  having  such  high  speeds  from  any  thermal  source, 

1  P.  R.,  10,  546  (1917). 
*P.  L.  P.  S.,  31,  1  (1918). 


162  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

however,  shows  it  to  be  negligibly  small.)  In  the  case  of  "brilliant 
arcs"  McLennan  considers  that  some  other  consideration  (e.  g.,  the 
part  played  by  positive  ions)  must  be  taken  into  account,  high 
speed  electrons  alone  being  inadequate  to  account  for  the  results. 

Compton1  investigated  whether  the  theory  of  successive  impacts 
was  adequate  to  explain  the  experimental  effects.  His  calcula- 
tions hinge  upon  the  average  time  interval  in  which  an  atom  can 
remain  in  an  abnormal  state.  Unfortunately,  this  is  not  known 
for  Hg,  but  use  was  made  of  Stark's  value,  6  X  10 ~7  sec.  for  hydro- 
gen, as  a  tentative  assumption.  His  conclusion  is  that  the  effect 
of  successive  impacts  may  in  some  cases  be  sufficient  to  account 
for  the  phenomena,  but  that,  in  general,  it  is  insufficient  to  account 
for  all  the  observed  phenomena.  The  cumulative  effect  of  ab- 
sorbed radiation  and  direct  impact  may  be  more  important.* 

Compton2  has  recently  made  quantitative  measurements  of  the 
amount  of  ionization  which  accompanies  radiation  in  He  produced 
by  electrons  whose  energy  is  between  the  radiating  potential  and 
the  ionizing  potential.  These  experiments  are  of  importance  in 
that  they  indicate  clearly  the  source  of  conflicting  results  on  the 
presence  of  ionization  below  the  ionizing  potential.  The  arrange- 
ment shown  in  fig.  20  was  used.  From  the  ratios  of  the  effects 
obtained  when  the  gauze  side  and  when  the  plate  side  of  the  cylinder 
faced  the  electron  stream  the  ratio  of  the  ionization  effect  to  the 
radiation  effect  could  be  obtained.  From  20  volts  to  25  volts 
(fig.  21)  this  ratio  was  a  constant  in  any  particular  case  and  de- 
creased abruptly  at  25  volts,  because  direct  ionization  set  in. 
The  following  are  the  results  obtained  at  various  pressures  with 
electron  currents  of  the  order  of  10 ~6  amperes: 


Pressure 
Mm. 

Ionization 

P.) 

Radiation  v"' 

0.0005 

0.055 

0:003 

0.176 

0.082 

0.333 

0.044 

0.410 

1.0 

2.22 

25.0 

11.4 

I  P.  R.,  15,  130,  476  (1920). 

*  In  view  of  the  suggestion  (discussed  above),  put  forward  by  Franck  and 
Knipping,  that  atoms  in  pure  helium  can  be  put  into  an  abnormal  state  by  collision 
with  20.45  volt  electrons,  and  show  no  tendency  to  return  to  the  normal  state,  it  may 
be  that  "successive  impacts"  will  account  for  much  more  of  the  cumulative  effects 
than  would  be  expected  on  the  assumption  that  the  abnormal  state  lasts  only  about 
6  X  10  ~7  sec. 

II  P.M.,  40,  553  (1920). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  163 

It  will  be  seen  that  at  higher  pressures  ionization  predominates, 
and  is  present  even  at  the  lowest  pressure.  When  the  pressure 
is  constant  and  the  electric  current  is  increased  the  ratio  also 
increases  but  not  much,  showing  that  the  cumulative  effect  of 
successive  impacts  is  not  so  important  as  the  cumulative  effect 
of  absorption  of  radiation  and  a  direct  impact. 

Horton  and  Bailey1  believe  that  their  results  show  that  when 
there  is  ionization  in  helium  below  25  volts,  it  is  due  to  ionization 
of  the  impurities  (e.  g.,  Hg)  by  the  20  volt  radiation  from  the 
helium.  While  their  experiments  certainly  show  that  apparently 
negligible  traces  of  impurities  produce  large  ionization  effects, 
Compton's  results  cannot  be  accounted  for  as  an  impurity  effect. 
Reasons  in  support  of  this  are  given  in  Compton's  paper.  Perhaps 
the  clearest  proof  of  the  existence  of  ionization  in  helium  below 
25.5  volts  is  the  striking  of  an  arc  in  the  purest  helium  at  20.2 
volts,  an  effect  which  is  too  big  to  be  accounted  for  by  traces  of 
impurities. 

Compton's  view  of  the  cumulative  effect  of  absorbed  radiation 
and  direct  impact  has  been  strengthened  by  some  very  recent 
experiments  of  Compton  and  Smyth2  in  which  it  was  found  that 
the  ionizing  potential  of  fluorescing  iodine  vapor  was  lower  than 
that  of  normal  iodine  vapor.  For  normal  iodine  vapor,  the  value 
was  about  10  volts,  while  for  iodine  vapor  made  fluorescent  by 
green  light  (corresponding  potential  2.3  volts),  the  value  was 
7.5  volts,  just  2.5  volts  less. 

An  interesting  illustration  of  cumulative  effects  appears  in  the 
recent  work  of  Compton,  Lilly  and  Olmstead.3  They  found  that 
in  the  helium  arc  all  the  ordinary  lines  appear  simultaneously 
when  the  arc  strikes.  As  we  have  seen,  in  order  that  the  arc 
shall  strike,  there  must  be  ionization,  and  when  this  takes  place 
below  25.5  volts,  it  cannot  be  anything  else  but  a  cumulative 
effect.  There  will  be  electrons  in  all  the  outer  orbits  dropping 
step  by  step  towards  the  inner  orbits  thus  emitting  lines  in  the 
visible  as  well  as  the  ultraviolet  region.  When  the  arc  is  intense, 
few  of  them  will  get  to  the  normal  position  as  the  frequent  electron 
collisions  re-eject  them.  The  enhanced  line  X  4686  belongs  to  the 
charged  helium  atom,  and  is  denoted  in  Bohr's  notation  as  4N 
{ 1/32  —  l/42j .  It  cannot  be  produced  until  a  helium  atom  already 

1  P.  M.,  40,  440  (1920). 

2  Science,  51,  571,  June  4  (1920). 

3  P.  R.,  16,  282  (1920). 


104 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


deprived  of  one  electron  is  disturbed.  As  the  formula  shows  it 
is  emitted  when  the  electron  drops  back  from  orbit  4  to  orbit  3,  in 
the  charged  atom  (fig.  26) .  It  can  be  produced  in  two  different  ways, 
the  first  being  found  when  the  electrons  in  the  electron  stream 
have  sufficient  energy  to  detach  both  electrons  from  the  atom. 
This  corresponds  to  the  ionizing  potential  80  volts  (see  table  on 
p.  142) .  In  agreement  with  this,  the  authors  found  that  the  enhanced 
line  X  4686  suddenly  appeared  when  the  potential  across  the  arc  was 
raised  to  80  volts  (the  gas  pressure  being  low  and  the  thermionic 
current  not  intense).  With  higher  pressure  and  more  intense 
currents,  the  line  becomes  visible  at  55  volts.  This  was  explained 
as  a  cumulative  effect,  one  example  of  which  is  the  removal  of  the 
first  electron  from  the  atom  and  a  collision  between  the  charged 
atom  and  an  electron  of  sufficient  energy  to  lift  the  remaining 

He,l>iu,m,  Atom, 


Ot-Jb/T    Numbers 

> 

y  45o 
II    1 

Energy       **,„  v  ,. 

\4686/ 

PT 

^                              5/5.73 

J 

-*                                    AA  T/7 

*n 

'TO  JU 

-^  4n-  67  »- 

electron  from  orbit  1  to  orbit  4  or  any  orbit  beyond.  (The  dia- 
gram shows  that  the  minimum  energy  required  is  50.72  volts.) 
Other  combinations  forming  cumulative  effects  are  given  in  the 
paper. 

Compton's  results  on  ionization  in  helium  between  the  ionizing 
and  radiating  potentials  open  up  interesting  considerations  as  to 
similar  phenomena  in  other  gases.  To  account  for  ionization  of 
helium  atoms  just  above  the  radiating  potential,  the  ionization 
must  be  a  second  "event"  in  the  recent  history  of  the  atom,  the 
first  being  the  absorption  of  energy,  20.4  volts,  to  make  the  atom 
an  abnormal  one,  either  through  an  earlier  impact,  or,  more  prob- 
ably, through  the  absorption  of  radiant  energy  from  neighboring 
atoms.  As  the  energy  required  to  remove  the  electron  completely 
from  an  abnormal  atom  is  25.3  —  20.4  =  4.9  volts,  there  is  energy 
to  spare  in  the  20.4  volt  electron  stream  for  the  "second  event." 
But  this  is  not  the  case  for  elements  in  which  the  ionizing  potential 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  165 

is  more  than  twice  the  radiating  potential.  We  may  cite  mercury 
vapor  (4.9  volts  and  10.4  volts),  and  still  more  to  the  point,  iodine 
vapor  (2.34  volts  and  10.1  volts).  Electrons  of  velocity  just 
above  4.9  volts  would  be  unable  to  ionize  a  mercury  atom  as  a 
result  of  two  "events;"  it  would  appear  that  three  "events"  would 
be  necessary.  Presumably  a  mercury  atom  cannot  acquire  any 
more  energy  than  4.9  volts,  through  the  absorption  of  4.9  volt 
radiation  (X  2536)  no  matter  how  intense  (as  radiation  of  different 
wave-length  corresponds  to  removal  from  2nd  to  3rd  orbit),  hence, 
it  would  appear  that  the  second  and  third  events  would  have  to 
be  successive  impacts.  Investigation  on  these  lines  should  lead 
to  important  results. 

COMPOUND   GASES 

Investigations  of  the  critical  potentials  for  compounds  should 
furnish  evidence  as  to  the  mode  in  which  atoms  are  bound  together 
in  a  compound.  The  first  critical  potentials  for  a  number  of 
compounds  were  determined  by  Hughes  and  Dixon1  (see  table  p.  136.) 
No  general  conclusions  could  be  drawn,  except  that  the  first  critical 
potentials  for  CH4,  CaHe,  CoH4,  C2H2  were  very  much  the  same. 
It  is  desirable  that  these  compounds  be  re-examined  on  the  lines 
of  the  best  of  the  recent  investigations  to  obtain  accurate  values 
for  the  radiating  and  ionizing  potentials  and  to  distinguish  between 
them. 

Several  important  theoretical  papers  by  Born2  and  Fajans8 
have  appeared  on  the  affinity  of  the  halogen  atoms  for  electrons, 
and  on  the  ionizing  potentials  of  HC1,  HBr  and  HI.  The  line 
of  argument  may  very  briefly  be  outlined  as  follows.  Born  regards 
a  crystal  such  as  KC1  as  being  held  together  by  forces  between 
the  positive  K  ion  and  the  negative  Cl  ion.  His  theory  gives  a 
value,  UKCI»  for  the  work  necessary  to  dissociate  the  crystal  into 
gaseous  K+  and  Cl-  ions.  This  final  stage  can  be  arrived  at  in 
another  way.  Imagine  the  crystal  to  be  dissociated  into  potassium 
(metal)  and  chlorine  gas,  and  then  the  potassium  to  be  vaporized 
and  afterwards  ionized  into  K+  and  free  electrons,  while  the  chlorine 
gas  C12  is  dissociated  into  atomic  Cl  and  afterwards  the  Cl  atoms 
unite  with  the  free  electrons  to  form  Cl-  ions.  The  energies  re- 
quired are,  respectively,  QKCI»  the  ordinary  heat  of  combination; 
DK,  the  heat  of  vaporization  of  K;  JK,  the  work  of  ionization  of  K; 

>  P.  R.,  10,  495  (1917). 

2  V.  d.  D.  P.  G.,  21,  13,  679  (1919). 

3  Ibid.,  21,  714  (1919). 


166  REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 

DC*,  the  heat  of  dissociation  of  C12;  and  Eci,  the  energy  given  out 
when  a  Cl  atom  combines  with  an  electron. 

UKCI  =  QKCI  +  DK  +  JK  -f-  Dcl  —  ECi 

All  the  values  are  known  except  ECi  which  turns  out  to  be  5.16 
volts.  This  means  that  energy  is  given  out  when  a  negative  Cl 
ion  is  formed,  i.  e.,  Cl-  is  stable. 

If  the  ionization  of  HC1  is  merely  the  disruption  of  the  molecule 
into  H+  and  CL-,  the  energy  necessary  to  effect  it,  JHCI>  can 
be  predicted  according  to  Born  and  Fajans.  The  decomposition 
of  HC1  into  Hg  and  CU  requires  an  amount  of  energy  QHCI  (the 
ordinary  heat  of  formation) ;  the  dissociation  of  H2  into  H  requires 
an  amount  of  energy  DH;  of  Cl2  into  Cl,  an  amount  DCi;  the 
ionization  of  atomic  H  into  H-f-  and  free  electrons  requires  an 
amount  JH  (given  by  the  ionizing  potential  13.52  volts) ;  and  the 
union  of  Cl  with  a  free  electron  corresponds  to  the  evolution  of 
heat  ECI-  Hence, 

JHCI  =  QHCI  +  DH  +  DC1  +  JH  —  ECi 

If  ECI  be  taken  to  be  5.16  volts,  then  JHci  will  have  a  value  of 
13.9  volts. 

This  was  tested  experimentally  by  Foote  and  Mohler1  who 
obtained  a  value  of  13.7  volts  for  the  ionizing  potential  of  HC1. 
This,  then,  may  be  regarded  as  confirming  the  view  of  Born  and 
Fajans  as  to  the  mechanism  of  ionization  of  HC1.  It  should  be 
mentioned  that  Hughes  and  Dixon2  found  a  critical  potential  in 
HC1  at  about  9.5  volts  (together  with  evidence  of  a  much  stronger 
ionization  at  about  13  volts)  which  may,  of  course,  be  a  radiating 
potential.  Foote  and  Mohler,  however,  state  that  they  have 
found  no  evidence  of  a  radiating  potential. 

A  useful  discussion  of  the  values  of  the  ionizing  potential  to  be 
expected  for  certain  compounds  has  been  given  by  Foote  and 
Mohler.3  The  compounds  considered  are  those  for  which  there 
is  reason  to  believe  that  ionization  takes  the  form  of  splitting  up 
the  molecule  into  positively  and  negatively  charged  atoms,  and  for 
which  there  is  sufficient  thermochemical  data  to  predict  the  value 
of  the  ionizing  potential.  The  results  are  arrived  at  on  lines  more 
pr  less  similar  to  those  used  in  the  last  paragraph  for  HC1. 

1  J.  A.  C.  S.,  42,  1832  (1920). 

2  P.  R.,  io,495  (1917). 

1  Jour.  Wash.  Acad.  Sci.,  10,  435  (1920). 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


167 


APPENDIX 

(Added  after  the  report  was  in  type — March,  1921) 
I.  The  principal  critical  potentials  for  many  metallic  vapors  have 
now  been  shown  to  be  associated,  through  the  quantum  relation 
(Ve  =  hv~)  with  lines  in  certain  series  in  their  spectra.  These  lines 
are  IS-  Ifr,  IS-  IP,  and  the  limit  IS,  for  the  metals  of  the  second 
column  of  the  periodic  table,  and  the  doublet  la  — ITT  and  the  cor- 
responding limit  la  for  the  alkali  metals.  Once  the  identity  of 
this  relation  is  admitted,  much  more  accurate  values  of  the  critical 
potentials  can  be  calculated  from  spectroscopic  data  than  can  be 
obtained  by  direct  experiment.  Professor  F.  A.  Saunders  kindly 
furnished  the  data  for  the  alkali  metals.  The  data  for  the  other 
metals  were  obtained  from  a  recent  paper  by  Mohler,  Foote,  and 
Meggers  (Bur.  Standards,  Sci.  Papers  No.  403,  1920).  It  should 
be  mentioned  that,  in  this  paper,  it  has  been  shown  experimentally 
that  each  element  (in  the  second  column  of  the  periodic  table)  in- 
vestigated, has  two  well-marked  radiating  potentials  and  one  ion- 
izing potential.  This  result  is  valuable  in  showing  the  uniformity  of 
the  elements  in  this  group  with  respect  to  the  critical  potentials, 
a  result  which  was  expected,  but  had  not  been  demonstrated.  (The 
calculated  potentials  are  given  to  four  significant  figures  only,  as 
h  and  e  are  not  known  to  less  than  1  in  1000.) 


Metal 

Series 

Wave-length 

Calculated  Potential 

loiizing 

Radiating 

Mercury 

IS 
IS—  Ip, 
IS—  IP 

1187.96 
2537  .48 
1849.60 

10.392 

4.865 
6.674 

Cadmium 

IS 
IS  —  Ip, 
IS  —  IP 

1378  .69 
3262.09 
2288.79 

8.954 

3.784 
5.394 

Zinc 

IS 
IS—  Ip, 
IS—  IP 

1319.98 
3076.88 
2139.33 

9.352 

4.012 
5.770 

Magnesium 

IS 
IS—  Ip, 
IS—  IP 

1621.72 
4572.65 
2853.06 

7.612 

2.700 
4.827 

Barium 

IS 

is  —  IP, 

IS—  IP 

2379.28 
7913.52 
5537.04 

5.188 

1.560 
2.229 

Strontium 

IS 
IS—  Ip, 
IS—  IP 

2176.94 
6894.45 
4608.61 

5.671 

1    791 
2.679 

168 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES 


Calcium 

IS 
IS—  IP 

2028.20 
6574.59 
4227.91 

6.087 

1.878 
2.920 

Caesium 

la 

la—  ITT 

3191.37 
f  8523  .33 
\  8945.82 

3.873 

f  1.447 
\  1  .378 

Rubidium 

la 
la—  IT 

2968.70 
f  7802.39 
\  7949.76 

4.154 

f  1  .580 
\  1.551 

Potassium 

Iff 

la—  lir 

2856.76 
f  7666.95 
\  7701  .  13 

4.317 

f  1  .608 
\  1.601 

Sodium 

la 
lal—v 

2412.84 
f  5891.78 
]  5897.76 

5.111 

f  2.093 
\  2.091 

Lithium 

la 
la—lw 

2299.67 
f  6709.94 
\6710.08 

5.362 

j  1.838 
\  1.838 

Zinc  ethyl 

Zinc  chloride 

Mercuric  chloride . 
Carbon  monoxide. 


R.  P. 

7  volts 


II.  Attention  is  drawn  to  the  following  recent  papers  bearing  on 
"Photo-Electricity." 

Foote  and  Mohler,  P.  R.,  17,  394(1921),  find  the  following  ionizing 
and  radiating  potentials. 

i.  P. 

12  volts 
12.9  volts 
12.1  volts 
10 . 1  volts 
14.3  volts 

Joly,  P.  M.,  41,  289  (1921)  and  Poole,  P.  M.,  41,  347  (1921),  dis- 
cuss the  application  of  photo-electricity  and  the  quantum  theory 
to  vision.  Experiments  on  the  photo-electric  effect  of  the  active 
materials  of  the  retina  are  described. 

Stebbins,  A.  P.  J.,  53,  105  (1921),  employed  a  photo-electric  cell 
to  study  the  fluctuations  of  the  star  Algol. 

Angerer,  P.  Z.,  22,  98  (1921),  made  use  of  a  photo-electric  cell 
to  study  the  after  glow  in  active  nitrogen.  In  connection  with  the 
point  raised  by  Elster  and  Geitel  (p.  107),  Angerer  was  able  to  study 
rather  rapid  changes  of  light  intensity  satisfactorily. 

Sir  J.  J.  Thomson,  P.  M.,  41,  526  (1921),  calculated  the  ionizing 
potentials  for  a  number  of  elements  on  the  basis  of  his  theory  of 
the  atom.  The  ratio  of  the  ionizing  potential  for  Li  to  that  for  H 
is  of  the  right  order;  the  ionizing  potential  for  Na  appears  to  be 
considerably  smaller  than  the  experimental  value.  The  theoretical 


REPORT  ON  PHOTO-ELECTRICITY:  A.  LL.  HUGHES  169 

values  for  O  and  N  cannot  be  compared  directly  with  the  experi- 
mental values  which  relate  to  O2  and  N2. 

A  good  discussion  of  critical  potentials  with  reference  to  the 
Bohr  type  of  atom  will  be  found  in  Sommerf eld's  "Atombau  und 
Spektrallinien"  (2nd  edition).  (The  writer  was  unable  to  get  a 
copy  in  time  to  make  use  of  it  in  drawing  up  the  report.) 

A  full  account  of  the  work  on  hydrogen  referred  to  on  p.  139 
under  the  names  of  Franck,  Knipping  and  Kruger  will  be  found  in 
an  article  by  Kruger  in  A.  d.  P.,  64,  288  (1921). 

Hodgman,  P.  R.,  17,  246  (1921),  gives  a  list  of  color  filters  (dyes 
in  gelatine)  with  transmission  ranges  (mainly  in  the  visible),  which 
may  prove  useful  in  photo-electric  experiments.  Data  regarding 
the  transmission  of  various  colored  glasses  for  use  in  securing  mono- 
chromatic light  when  used  in  conjunction  with  the  mercury  arc, 
or  the  discharge  in  hydrogen  or  helium  will  be  found  in  Technologic 
Paper  No.  148,  1920  of  the  Bureau  of  Standards. 


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